Documentation issues to fix for beta

[rjrjr]

Services is basically undocumented, and has this stale bit in it.

  /**
   * Sets up any services associated with the key, and make them accessible via the context.
   * Typically this means returning a new context that wraps the given one.
   */
  public abstract void bindServices(@NonNull Services.Binder services);

I'll add other doc issues to this ticket as I notice them.

[dcow]

It would be good to know the memory management expectations around when bind/teardown are called. From what I could gather, services are bound when a key is added to the stack and torn down when the key leaves the stack (not when it's no longer the top). If services are meant to replace flow-path and be the integration point for a scoping solution like mortar, then some sort of documentation around the behavior is needed.