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)?

thanks @twoertwein  Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.2.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 2bcd31343a072a03913fea151a33857e420cd49b
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #39492: DOC: Document how encoding errors are handled'
```

4. Push to a named branch :

```
git push YOURFORK 1.2.x:auto-backport-of-pr-39492-on-1.2.x
```

5. Create a PR against branch 1.2.x, I would have named this PR:

> "Backport PR #39492 on branch 1.2.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 @meeseeksdev backport 1.2.x Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.2.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 2bcd31343a072a03913fea151a33857e420cd49b
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #39492: DOC: Document how encoding errors are handled'
```

4. Push to a named branch :

```
git push YOURFORK 1.2.x:auto-backport-of-pr-39492-on-1.2.x
```

5. Create a PR against branch 1.2.x, I would have named this PR:

> "Backport PR #39492 on branch 1.2.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                