DOC: update the DataFrame.head()  docstring
- Improved summary details
- Added description of last in see-also

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.DataFrame.head)  ######################
################################################################################

Return the first n rows.

This function returns rows for the object based on the underlying
index. It is useful for quickly testing if your object has the
right type of data in it.

Parameters
----------
n : int, default 5
    Number of rows to select.

Returns
-------
obj_head : type of caller
    The first n rows of the caller object.

See Also
--------
pandas.DataFrame.tail
    Returns the last n rows.

Examples
--------
>>> df = pd.DataFrame({'animal':['alligator', 'bee', 'falcon', 'lion',
...                    'monkey', 'parrot', 'shark', 'whale', 'zebra']})
>>> df
      animal
0  alligator
1        bee
2     falcon
3       lion
4     monkey
5     parrot
6      shark
7      whale
8      zebra

Viewing the first 5 lines

>>> df.head()
      animal
0  alligator
1        bee
2     falcon
3       lion
4     monkey

Viewing the first n lines (three in this case)

>>> df.head(3)
      animal
0  alligator
1        bee
2     falcon

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

Docstring for "pandas.DataFrame.head" 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.
Thanks!