Document persistent connection behavior

The legacy driver had documentation for the various connection handling behaviors across multiple versions (see: here). We should have similar documentation for the new driver.

In particular, this will need to cover:

• Persistent sockets (but not topologies) in PHPC versions before 1.2.0, which used PHP streams for socket handling and SSL
• No connection persistence in HHVM versions before 1.2.0, which used libmongoc for socket handling and SSL
• Persistent mongoc_client_t (sockets and topolgies) in PHPC and HHVM 1.2.0+, which use libmongoc for socket handling and SSL

Additionally, this documentation should discuss how persistent connections are hashed in 1.2.0+ and explain reasoning for not providing the same API found in the legacy driver (e.g. a close() method).