* src/engine/nonblocking/nonblocking-abstract-semaphore.vala: Renamed to
  nonblocking-lock.vala, rename class from AbstractSemaphore to Lock and
  update subclasses, since it is used as a basis for a number of
  different lock implementations. Make two getter methods into read-only
  properties. Fill out doc comments to be much more comprehensive.

* src/engine/nonblocking/nonblocking-mutex.vala (Mutex): Provide a
  high-level API and update documentation comments.

* src/engine/nonblocking/nonblocking-queue.vala (Queue): Add to doc
  comments.

* src/engine/nonblocking/nonblocking-variants.vala: Actually document
  how each of the variants behave.