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.

[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?

[kamil-tekiela]

@lacatoire is right. This change was done to multiple functions and such such should be documented consistently. php/php-src@90eeca2#diff-28e30bf65ac518b331e0315af123e060f35e9053d7c05b68b5bcbcd1be080ba4

Since the change was already documented in one place, all you need to do is take it out and put it into a new snippet (language-snippets.ent) and then use that snippet for all the changed functions. Both the Errors/Exceptions and Changelog sections need to be adjusted.