[Sync EN] socket_select(): document key preservation, and correct both select pages (#5829)

Commit: `https://github.com/php/doc-en/commit/3e6082f609ead63cfa7b33a06d8e0c723d51bc65`
PR: `https://github.com/php/doc-en/pull/5829`

**Fichiers FR à mettre à jour**
- `reference/sockets/functions/socket-select.xml`
- `reference/stream/functions/stream-select.xml`
