add descriptions for all public APIs

[jeffbcross]

[katowulf]

FYI - all methods in FirebaseObjectObservable and FirebaseListObservable are documented in api-reference.md. Looks like $ref and possibly a couple of others are needed in the guide docs here and here.

Looks like Authentication could also use some additions. Are we aware of any other outstanding classes that need to get documented?

[katowulf]

@mukesh51 want to take a look at this one?

[mukesh51]

@katowulf Sure. I'll start working on this.

[mukesh51]

@katowulf I've added a section AngularFireAuth. Please review and let me know, if it looks ok or needs to be amended.

[katowulf]

I think that with Mukesh's changes, we have now added docs for all public APIs.