mb_strlen & friends error section for $encoding

[nielsdos]

https://www.php.net/manual/en/function.mb-strlen.php mentions in the error section that a warning may be raised with an invalid encoding. But that's not what happens: https://3v4l.org/E9s0i. A ValueError is raised instead.
For some other pages, the error condition for $encoding is also not specified. I only had a quick look instead of composing a list of all the pages that lack one.

[Girgias]

The ValueError is likely raised only as of PHP 8.0, and a warning was emitted during 7.X

[nielsdos]

Yep just checked, that's right. So we need a changelog entry as well, both the changelog and the ValueError message would be nice as entities I guess.

[Girgias]

Yup, a lot of changelog entries are already entities :D for the ValueError it would be better to just have the paragraph be an entity