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.
is this overlapping with #20099 ? Hello @lastone9182! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on March 12, 2018 at 21:10 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20100?src=pr&el=h1) Report
> Merging [#20100](https://codecov.io/gh/pandas-dev/pandas/pull/20100?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/52cffa3b3b2a510c30ed7f8cc8525c03d62e9130?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20100/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20100?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #20100   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files         150      150           
  Lines       49149    49149           
=======================================
  Hits        45083    45083           
  Misses       4066     4066
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <ø> (ø)` | :arrow_up: |
| #single | `41.85% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20100?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20100/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.84% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20100?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20100?src=pr&el=footer). Last update [52cffa3...8058931](https://codecov.io/gh/pandas-dev/pandas/pull/20100?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
