* Annotate Transaction and WriteBatch methods as discardable
These methods exist to allow chaining if that's useful but chaining
isn't expected or required.
Note that Objective-C methods in these types are implicitly
@discardableResult and don't need any extra help.
* No need to explicitly discard results
* Fix typos in FIRCollectionReference docs
* Add support for CollectionReference.addDocument(from:)
