Held Write Acks Changelog entries

As per https://firebase.google.com/support/release-notes/js

Author: schmidt-sebastian

firebase-firestore/CHANGELOG.md

@@ -1,3 +1,12 @@
+# Unreleased
+- [changed] Changed the internal handling for locally updated documents that
+  haven't yet been read back from Cloud Firestore. This can lead to slight
+  behavior changes and may affect the `SnapshotMetadata.hasPendingWrites()`
+  metadata flag.
+- [changed] Eliminated superfluous update events for locally cached documents
+  that are known to lag behind the server version. Instead, we buffer these
+  events until the client has caught up with the server.
+
 # 17.1.1
 - [fixed] Fixed an issue where the first `get()` call made after being offline
   could incorrectly return cached data without attempting to reach the backend.