[Sync EN] Document boolean support for allowed_classes in unserialize (#4768)

Commit: `https://github.com/php/doc-en/commit/b2acdf5697fa3c189f24c3b0147bef57cb8a6c32`
PR: `https://github.com/php/doc-en/pull/4768`

**Archivos ES a actualizar**
- `appendices/migration84/incompatible.xml`
- `reference/var/functions/unserialize.xml`

