Fix GH-1132: Document mb_strlen invalid encoding behavior change in P…

[GabrielCordeiroBarrosoTeles]

Fixes #1132

Documents that passing an invalid encoding to mb_strlen() had undefined behavior before PHP 8.0 (could return false) and that as of PHP 8.0 a ValueError is thrown instead, as suggested in the issue discussion.

[jordikroon]

Please use <exceptionname>ValueError</exceptionname> instead.

And Previously, &false; was returned. makes more sense here since prior to 8.0, false was always returned on error.

https://github.com/php/php-src/blob/PHP-7.4.33/ext/mbstring/mbstring.c#L2503

[GabrielCordeiroBarrosoTeles]

Done, @jordikroon . Switched to <exceptionname>ValueError</exceptionname> and updated to "Previously, &false; was returned." to match the PHP 7.4 source. Thanks.

[lacatoire]

Hi, i checked in php-src and the ValueError for invalid encodings comes from php_mb_get_encoding() (commit [21227dc1f19](php/php-src@21227dc1f19e
a9007ecfc6929261f16900359c4d), which is shared across many most functions, not just mb_strlen.

Currently only mb_convert_encoding and mb_encoding_aliases document this 8.0.0 change. Would it make sense to extend this PR to cover the other functions as well?