These doc comments are very minor, but I felt like adding something and I didn't know what to put. The doc comments are for the module and for pub elements.

This way, RwLockReadGuard isn't being juggled anywhere, and the bounds of the session MutexGuard is limited to the FnOnce explicitly passed to with_session (less room for accidents, nice!)

This approach was inspired by the source code of RegionMap of Bamboo (https://gitlab.com/macmv/bamboo ref 8e32b54a5e7a69986339fea465d4afcc20425dd7), which uses a similar RwLock<HashMap<_, Mutex<_>>>. I like this approach because it allows me to provide access to the Session without having to clone it and allows the MutexGuard and RwLockReadGuard to be held as the user needs without allowing the user to access the RwLockReadGuard and poke around the rest of the entries in the HashMap.

Also remove Arc from the HashMap

Changes in this commit have somewhat mollified my code-smell-o-meter.