Backporting reviewer suggested comments from cl/214509458 (#1904)

This is the iOS port of
firebase/firebase-android-sdk#49

Author: rsgowman

Firestore/CHANGELOG.md

@@ -8,11 +8,10 @@
 # v0.13.4
 - [fixed] Fixed an issue where the first `get()` call made after being offline
   could incorrectly return cached data without attempting to reach the backend.
-- [changed] Changed `get()` to only make 1 attempt to reach the backend before
-  returning cached data, potentially reducing delays while offline. Previously
-  it would make 2 attempts, to work around a backend bug.
-- [fixed] Fixed an issue that caused us to drop empty objects from calls to
-  `setData(..., merge:true)`.
+- [changed] Changed `get()` to only make one attempt to reach the backend before
+  returning cached data, potentially reducing delays while offline.
+- [fixed] Fixed an issue that caused Firebase to drop empty objects from calls
+  to `setData(..., merge:true)`.
 
 # v0.13.3
 - [changed] Internal improvements.