DOC: Updating str_pad docstring
- [x ] tests added / passed
- [ x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Pad documentation was describing a different behaviour, so I fixed it and added examples.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22567?src=pr&el=h1) Report
> Merging [#22567](https://codecov.io/gh/pandas-dev/pandas/pull/22567?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/98fb53ca2d64658749e9f182b391002ef3500c1b?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/22567/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22567?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22567   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         169      169           
  Lines       50787    50787           
=======================================
  Hits        46745    46745           
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <ø> (ø)` | :arrow_up: |
| #single | `42.29% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22567?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/22567/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22567?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/22567?src=pr&el=footer). Last update [98fb53c...8337a53](https://codecov.io/gh/pandas-dev/pandas/pull/22567?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I messed up. It continues here: #22570 