Add names for the mutex requirements

[sdutoit]

In particular, Mutex, TimedMutex, SharedMutex. There was a request that we don't, in fact, name the basic requirement Mutex so that it is easier to search for.

See also #202.

[jensmaurer]

See #202. The mutex requirements are now referenced using the obvious prose names. There's no obvious non-confusing alternative for "mutex requirements", so I left it at that. Specific suggestions welcome.