firebase-ios-sdk

Fixes for the Codable API (#3734)

* 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:)