DOC: update the pandas.Series.str.split docstring
Following up from discussion on #20282 

- [x] PR title is "DOC: update the pandas.Series.str.split docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py pandas.Series.str.split docstring`
- [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 pandas.Series.str.split docstring`

```
################################################################################
##################### Docstring (pandas.Series.str.split)  #####################
################################################################################

Split strings around given separator/delimiter.

Split each string in the caller's values by given
pattern, propagating NaN values. Equivalent to :meth:`str.split`.

Parameters
----------
pat : str, optional
    String or regular expression to split on.
    If not specified, split on whitespace.
n : int, default -1 (all)
    Limit number of splits in output.
    ``None``, 0 and -1 will be interpreted as return all splits.
expand : bool, default False
    Expand the splitted strings into separate columns.

    * If ``True``, return DataFrame/MultiIndex expanding dimensionality.
    * If ``False``, return Series/Index, containing lists of strings.

Returns
-------
split : Series/Index or DataFrame/MultiIndex of objects
    Type matches caller unless ``expand=True`` (return type is DataFrame or
MultiIndex)

Notes
-----
The handling of the `n` keyword depends on the number of found splits:

- If found splits > `n`,  make first `n` splits only
- If found splits <= `n`, make all splits
- If for a certain row the number of found splits < `n`,
  append `None` for padding up to `n` if ``expand=True``

Examples
--------
>>> s = pd.Series(["this is good text", "but this is even better"])

By default, split will return an object of the same size
having lists containing the split elements

>>> s.str.split()
0           [this, is, good, text]
1    [but, this, is, even, better]
dtype: object
>>> s.str.split("random")
0          [this is good text]
1    [but this is even better]
dtype: object

When using ``expand=True``, the split elements will expand out into
separate columns.

For Series object, output return type is DataFrame.

>>> s.str.split(expand=True)
      0     1     2     3       4
0  this    is  good  text    None
1   but  this    is  even  better
>>> s.str.split(" is ", expand=True)
          0            1
0      this    good text
1  but this  even better

For Index object, output return type is MultiIndex.

>>> i = pd.Index(["ba 100 001", "ba 101 002", "ba 102 003"])
>>> i.str.split(expand=True)
MultiIndex(levels=[['ba'], ['100', '101', '102'], ['001', '002', '003']],
       labels=[[0, 0, 0], [0, 1, 2], [0, 1, 2]])

Parameter `n` can be used to limit the number of splits in the output.

>>> s.str.split("is", n=1)
0          [th,  is good text]
1    [but th,  is even better]
dtype: object
>>> s.str.split("is", n=1, expand=True)
        0                1
0      th     is good text
1  but th   is even better

If NaN is present, it is propagated throughout the columns
during the split.

>>> s = pd.Series(["this is good text", "but this is even better", np.nan])
>>> s.str.split(n=3, expand=True)
      0     1     2            3
0  this    is  good         text
1   but  this    is  even better
2   NaN   NaN   NaN          NaN

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

Errors found:
	See Also section not found
```

@jorisvandenbossche @WillAyd 
@mananpal1997 Thanks!

Additional comment: on line 1121, the indentation should be the same as the line above, can you fix that as well? (cannot comment on that line :-))

@WillAyd did you have a proposal to write the Returns section more clear? Nice job @mananpal1997. I suggest the Return type line just say `Series, DataFrame, Index or MultiIndex` and the description for the return should say `Type matches caller unless ``expand=True`` (see Notes)`. Then in the notes say something like `if using ``expand=True``,  Series and Index callers will return DataFrame and MultiIndex objects, respectively`.
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20307?src=pr&el=h1) Report
> Merging [#20307](https://codecov.io/gh/pandas-dev/pandas/pull/20307?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7169830040255c1073f99e2ee8817596a67b1cd5?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

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```diff
@@           Coverage Diff           @@
##           master   #20307   +/-   ##
=======================================
  Coverage    91.7%    91.7%           
=======================================
  Files         150      150           
  Lines       49165    49165           
=======================================
  Hits        45087    45087           
  Misses       4078     4078
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.09% <ø> (ø)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20307?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20307/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20307/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.84% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20307?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/20307?src=pr&el=footer). Last update [7169830...27548e8](https://codecov.io/gh/pandas-dev/pandas/pull/20307?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Added a see also. Thanks @mananpal1997.