Add documentation for explicitly setting Object IDs

[mcsimps2]

You can currently set it as <doc>.id = <ObjectID> and then do <doc>.save(). You may also be able to set in in the constructor. This behavior should be documented.

[mcsimps2]

In general, there should be documentation about the instance fields accessible, such as the id, coll_ref, coll_name, getReference, and getSnapshot

[mcsimps2]

Also, add documentation about how to get the id of an object without actually fetching it by using the corresponding private instance field. Perhaps there should be a method or property for this.