Currently, there is a global read/write lock. MongoDB does extensive yielding and tracking of on vs. off disk to mitigate the effects of this. In subsequent versions (2.2) , we'll be lowering the scope of this lock as well.
No, a read write lock is a lock designed so that multiple readers can hold the lock, but only a single writer can hold the lock. That way readers know no write is taking place(as many reads as you like can take place), and writers know they have exclusive access.
Is this page new as of this past week [edit: its new as of today]? Either way, every large software project should always try to have some standard page for this sort of information, so kudos to them for now having one (and its a visible link on the homepage too).