Added docs for the providers

lib/src/resources/firebase_storage_provider.dart

@@ -6,9 +6,14 @@ import 'package:uuid/uuid.dart';
 export 'package:firebase_storage/firebase_storage.dart'
     show StorageUploadTask, StorageTaskSnapshot;
 
+/// A connection to the firebase sotrage bucket.
 class FirebaseStorageProvider {
+  /// The reference to the root path of the storage bucket.
   final StorageReference _storage;
+
+  /// An instance of a uuid generator.
   final Uuid _uuid;
+
   // [FirebaseStorage] and [Uuid] instances can be injected for testing purposes.
   // Don't remove.
   FirebaseStorageProvider([StorageReference storage, Uuid uuid])

lib/src/resources/firestore_provider.dart

@@ -64,7 +64,7 @@ class FirestoreProvider {
     );
   }
 
-  /// Creates a new instance of a user in Firestore.
+  /// Creates a new user in Firestore.
   Future<void> createUser(UserModel user, String uid) async {
     try {
       final dataMap = user.toFirestoreMap();

lib/src/resources/google_sign_in_provider.dart

@@ -6,7 +6,7 @@ import 'package:rxdart/rxdart.dart';
 
 export 'package:firebase_auth/firebase_auth.dart' show FirebaseUser;
 
-/// A Google authentication provider.
+/// Google authentication provider.
 ///
 /// Connects to both Google and Firebase to authenticate a user.
 class GoogleSignInProvider {