DOC: update the docstrings of Interval and IntervalMixin
Improved the docstrings of Interval (the class) and the following methods
of IntervalMixin:
* closed_right
* closed_left
* open_right
* open_left

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [X] PR title is "DOC: update the <your-function-or-method> docstring"
- [ ] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [X] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [X] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
################################################################################
######################### Docstring (pandas.Interval)  #########################
################################################################################

Immutable object implementing an Interval, a bounded slice-like interval.

.. versionadded:: 0.20.0

Parameters
----------
left : value
    Left bound for the interval.
right : value
    Right bound for the interval.
closed : {'right', 'left', 'both', 'neither'}, default 'right'
    Whether the interval is closed on the left-side, right-side, both or
    neither.
    A closed interval (in mathematics denoted by square brackets)
    contains the edge points, i.e. the closed interval `[1, 5]` consists
    of the points `1, 2, 3, 4, 5`.
    An open interval (in mathematics denoted by parentheses) does not
    contain the edge points, i.e. the open interval `(1, 5)` consists of
    the points 2, 3, 4.
    Intervals can also be half-open or half closed, i.e.
    `[1,5) = 1, 2, 3, 4` and `(1, 5] = 2, 3, 4, 5`. See also the examples
    section below.

Examples
--------
>>> iv = pd.Interval(left=0, right=5)
>>> iv
Interval(0, 5, closed='right')
>>> 2.5 in iv
True
>>> 0 in iv
False
>>> 5 in iv
True

>>> year_2017 = pd.Interval(pd.Timestamp('2017-01-01'),
...                         pd.Timestamp('2017-12-31'), closed='both')
>>> pd.Timestamp('2017-01-01 00:00') in year_2017
True

See Also
--------
IntervalIndex : An Index of Interval objects that are all closed on the
                same side.
cut : Convert arrays of continuous data into bins of Series/Categoricals
      of Interval.
qcut : Convert arrays of continuous data into bins of Series/Categoricals
       of Interval based on quantiles.

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        Errors in parameters section
                Unknown parameters {'left', 'right', 'closed'}
        No returns section found
(venv) [akoeltringer@client7 pandas]$ python scripts/validate_docstrings.py pandas.Interval.closed_left

################################################################################
################### Docstring (pandas.Interval.closed_left)  ###################
################################################################################

Check if the interval is closed on the left side.

For the meaning of `closed` and `open` see :class:`~pandas.Interval`.

Returns
-------
bool
    `True` if the Interval is closed on the left-side, else `False`.

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        See Also section not found
        No examples section found
(venv) [akoeltringer@client7 pandas]$ python scripts/validate_docstrings.py pandas.Interval.closed_right

################################################################################
################### Docstring (pandas.Interval.closed_right) ###################
################################################################################

Check if the interval is closed on the right side.

For the meaning of `closed` and `open` see :class:`~pandas.Interval`.

Returns
-------
bool
    `True` if the Interval is closed on the right-side, else `False`.

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        See Also section not found
        No examples section found
(venv) [akoeltringer@client7 pandas]$ python scripts/validate_docstrings.py pandas.Interval.open_left

################################################################################
#################### Docstring (pandas.Interval.open_left)  ####################
################################################################################

Check if the interval is open on the left side.

For the meaning of `closed` and `open` see :class:`~pandas.Interval`.

Returns
-------
bool
    `True` if the Interval is open on the left-side, else `False`.

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        See Also section not found
        No examples section found
(venv) [akoeltringer@client7 pandas]$ python scripts/validate_docstrings.py pandas.Interval.open_right

################################################################################
#################### Docstring (pandas.Interval.open_right) ####################
################################################################################

Check if the interval is open on the right side.

For the meaning of `closed` and `open` see :class:`~pandas.Interval`.

Returns
-------
bool
    `True` if the Interval is open on the right-side, else `False`.

################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        See Also section not found
        No examples section found
```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.

* It is not clear why the error for ``pandas.Interval`` (``Unknown parameters {'left', 'right', 'closed'}``) arises - respondents to this question on Gitter were not sure either
* the ``closed_right``, ``closed_left``, ``open_right`` and ``open_left`` methods have no 'examples' and no 'see also' section because for that the reader is referred to the ``Interval`` class.