DOC: update the dtypes/ftypes docstring (Seoul)
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:

```
# paste output of "scripts/validate_docstrings.py <your-function-or-method>" here
# between the "```" (remove this comment, but keep the "```")


################################################################################
##################### Docstring (pandas.DataFrame.dtypes)  #####################
################################################################################

Return the dtypes in this object.

Notes
-----
It returns a Series with the data type of each column.
If object contains data multiple dtypes in a single column,
dtypes will be chosen to accommodate all of the data types.
``object`` is the most general.

Examples
--------
>>> df = pd.DataFrame({'f': pd.np.random.rand(3),
...                    'i': 1,
...                    'd': pd.Timestamp('20180310'),
...                    'o': 'foo'})
>>> df.dtypes
f           float64
i             int64
d    datetime64[ns]
o            object
dtype: object

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

Errors found:
	No extended summary found
	No returns section found
	See Also section not found


################################################################################
##################### Docstring (pandas.DataFrame.ftypes)  #####################
################################################################################

Return the ftypes (indication of sparse/dense and dtype)
in this object.

Notes
-----
Sparse data should have the same dtypes as its dense representation

See Also
--------
dtypes, SparseDataFrame

Examples
--------
>>> arr = pd.np.random.randn(100, 4)
>>> arr[arr < .8] = pd.np.nan
>>> pd.DataFrame(arr).ftypes
0    float64:dense
1    float64:dense
2    float64:dense
3    float64:dense
dtype: object
>>> pd.SparseDataFrame(arr).ftypes
0    float64:sparse
1    float64:sparse
2    float64:sparse
3    float64:sparse
dtype: object

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

Errors found:
	No summary found (a short summary in a single line should be present at the beginning of the docstring)
	No returns section found
	Missing description for See Also "dtypes" reference
	Missing description for See Also "SparseDataFrame" reference

```

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.

Lastly, I left errors already occurred in the previous version without changes.