Update CHANGELOG.md to mention limbo resolution throttling

This change to the change log was forgotten in the PR that added throttling: #2790

Author: dconeybe

packages/firestore/CHANGELOG.md

@@ -1,4 +1,9 @@
 # Unreleased
+- [changed] Limbo resolution now attempts to resolve a maximum of 100 documents
+  at a time, instead of resolving all at once (#2790). This addresses an issue
+  where the server can fail with "resource exhausted" errors if a large number
+  of documents are in limbo (e.g. 15,000), which can lead to the pathological
+  client behavior seen in #2683.
 - [changed] Changed the in-memory representation of Firestore documents to
   reduce memory allocations and improve performance. Calls to 
   `DocumentSnapshot.getData()` and `DocumentSnapshot.toObject()` will see