DOC: update the pd.Index.asof docstring
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"
- [x] 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.Index.asof)  ########################
################################################################################

Return the latest index label up to and including the passed label.

For sorted indexes, return the index label that is the latest among
the labels that are not later than the passed index label.

Parameters
----------
label : object
    The label up to which the method returns the latest index label.

Returns
-------
object : The index label that is the latest as of the passed label,
    or NaN if there is no such label.

See also
--------
Index.get_loc : `asof` is a thin wrapper around `get_loc`
    with method='pad'.

Examples
--------
13 is the latest index label up to 14.

>>> pd.Index([13, 18, 20]).asof(14)
13

If the label is in the index, the method returns the passed label.

>>> pd.Index([13, 18, 20]).asof(18)
18

If all of the labels in the index are later than the passed label,
NaN is returned.

>>> pd.Index([13, 18, 20]).asof(1)
nan

If the index is not sorted, an error is raised.

>>> pd.Index([13, 20, 18]).asof(18)
Traceback (most recent call last):
ValueError: index must be monotonic increasing or decreasing

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

Docstring for "pandas.Index.asof" correct. :)

```

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.

