docs/release-notes: add entry for rpc graph cache

docs/release-notes/release-notes-0.14.0.md

@@ -515,6 +515,11 @@ messages directly. There is no routing/path finding involved.
   buffer each time we decrypt an incoming message, as we
   recycle these buffers in the peer.
 
+* [The `DescribeGraph` and `GetNetworkInfo` calls have been
+  optimized](https://github.com/lightningnetwork/lnd/pull/5873) by caching the
+  response periodically, or using the new channel graph cache directly.  This
+  should significantly cut down on the garbage these two calls generate.
+
 ## Log system
 
 * [Save compressed log files from logrorate during