DOC: Document how encoding errors are handled
xref #39450

This should probably go in 1.2.2.

<1.2.0: `engine=c` and `engine=python` handled encoding errors differently when `encoding=None` (python: "replace", c: "strict"). The `engine=c` further supported to ignore lines (`skiprows`) with encoding errors iff `encoding=None`.

1.2.0 (shared file opening code between `engine=c` and `engine=python`): both engine mistakenly used "strict" when `encoding=None`. `engine=c`+`encoding=None` can no longer skip encoding errors.

1.2.1: both engine use "replace" when `encoding=None`. Both engines can ignore encoding errors in skipped rows iff `encoding=None`.

Option for 1.3: default to "strict" but expose `errors` in `read_csv` (and other read calls that might need it)?
