Clarify extra bandwidth usage

Author: wilhuff

Firestore/CHANGELOG.md

@@ -3,7 +3,8 @@
 # v0.12.6
 - [fixed] Fixed an issue where queries returned fewer results than they should,
   caused by documents that were cached as deleted when they should not have
-  been (#1548).
+  been (#1548). Some cache data is cleared and so clients may use extra
+  bandwidth the first time they launch with this version of the SDK.
 
 # v0.12.5
 - [changed] Internal improvements.