DOC: specify regex=True in str.replace
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

ref #36695

When passing a callable repl, str.replace will raise if regex is False. So it seems to me that in the future, when the default is changed, we should be ignoring the regex argument completely (and update the documentation to that effect). This is slightly magical, but I think better than having a default that raises.

If this is the correct way forward, then we don't need to warn when repl is callable.

cc @dsaxton @jorisvandenbossche
