DOC: update the str_replace docstring
Remove claim in `str_replace` docstring of equivalence to base Python `str.replace` -- the former does regex parsing while the latter does not.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20618?src=pr&el=h1) Report
> Merging [#20618](https://codecov.io/gh/pandas-dev/pandas/pull/20618?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6d610a4d9393c0d0335267dc3252ccabb9e51e43?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20618/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20618?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20618      +/-   ##
==========================================
+ Coverage   91.82%   91.84%   +0.02%     
==========================================
  Files         153      153              
  Lines       49256    49256              
==========================================
+ Hits        45227    45239      +12     
+ Misses       4029     4017      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20618?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20618/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20618/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20618?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/20618?src=pr&el=footer). Last update [6d610a4...c8102f4](https://codecov.io/gh/pandas-dev/pandas/pull/20618?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Aren't they equivalent if `regex=False` ? If nothing else, the relevance of `regex=False` should be explicit. E.g.

"""
Replace occurrences of pattern/regex in the Series/Index with some other string. Equivalent to :func:`re.sub` when regex=True, or :meth:`str.replace` when `regex=False`.
""" @gsganden feel free to open a new PR with your last suggestion (being explicit about when it's equivalent to `str.replace` and when to `re.sub`). Closing this one, as I think it's helpful for users to know it's equivalent to `str.replace`.