DOC: Expand docstrings for head / tail methods
This is just a first draft of some documentation changes to head and tail methods.

From EuroPython sprint.
Hello @yosukeBayashi! Thanks for updating the PR.



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

##### Comment last updated on August 21, 2017 at 04:39 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=h1) Report
> Merging [#16941](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ad24759871ea43131711cfce1e5fc69c06d82956?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16941/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16941      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.02%     
==========================================
  Files         161      161              
  Lines       49306    49306              
==========================================
- Hits        44868    44859       -9     
- Misses       4438     4447       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <ø> (ø)` | :arrow_up: |
| #single | `40.19% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.29% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16941?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/16941?src=pr&el=footer). Last update [ad24759...9c04831](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=h1) Report
> Merging [#16941](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/58d872903449b8a29237288ade6227cdb280fe18?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16941/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16941      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.02%     
==========================================
  Files         162      162              
  Lines       49567    49567              
==========================================
- Hits        45123    45114       -9     
- Misses       4444     4453       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.79% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `92.03% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16941?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/16941?src=pr&el=footer). Last update [58d8729...50b9bb7](https://codecov.io/gh/pandas-dev/pandas/pull/16941?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 And don't worry about the pycodestyle bot. We're testing it out. can you update according to comments pls rebase / update. @gfyoung if you can do this if OP doesn't respond in a while. > @gfyoung if you can do this if OP doesn't respond in a while.

Sure thing! @jreback : I'm going to assign this to myself.  Feel free to ping if this hasn't been completed before the `0.21` release, and I'll carry it home. @jreback : All review comments addressed.  PTAL. Thanks @yosukeBayashi and @gfyoung !