Add clearer docs around forking and threading concerns

The PHPC documentation warns users about forking (e.g. Manager::__construct(), but we don't go into significant detail about why driver objects cannot be shared across processes/threads. PHPC only refers to forking (not threads), and neither forking nor threads are discussed in the PHPLIB docs at all.

We would do well to summarize the explanation from PHPLIB-723 and ensure this is communicated clearly in documentation for both projects. Additionally, we can discuss how disableClientPersistence can be utilized (e.g. constructing Managers after spawning processes or threads).