[Sync EN] ext/simplexml: Document that get methods and string casts no longer implicitly rewind (#5737)

Commit: `https://github.com/php/doc-en/commit/418ec505128707215c5c99ad4a66a88bfd84a0ff`
PR: `https://github.com/php/doc-en/pull/5737`

**Fichiers FR à mettre à jour**
- `reference/simplexml/simplexmlelement.xml`
- `reference/simplexml/simplexmlelement/rewind.xml`
