[Sync EN] ext/pcre: document variable-length lookbehind and longer subpattern names (PHP 8.4) (#5739)

[lacatoire]

Syncs the Spanish translation with php/doc-en#5739 (commit c083c88).

• reference/pcre/pattern.syntax.xml:
  • new paragraph: as of PHP 8.4.0 (PCRE2 10.44) a subpattern name may be up to 128 characters, previously 32;
  • lookbehind assertions are documented as variable-length, with the fixed-length restriction moved to a separate paragraph as historical information for versions prior to 8.4.0;
  • the two touched paragraphs use simpara, as in EN.
• EN-Revision updated.

Fixes: #983