doc: add GetReplicationDestinationInfo to pending release notes

Document the new GetReplicationDestinationInfo RPC feature in the
pending release notes for v3.18.

This RPC enables DR orchestrators to map source volume/volume group IDs
to destination IDs across mirrored clusters when pools have different
IDs, supporting both single volumes and volume groups with pool-based
mapping configuration.

Signed-off-by: Rewant Soni <resoni@redhat.com>

Author: rewantsoni

PendingReleaseNotes.md

@@ -4,4 +4,16 @@
 
 ## Features
 
+1. Added `GetReplicationDestinationInfo` RPC to map source volume/volume
+  group IDs to destination IDs across mirrored clusters. This enables DR
+  orchestrators to discover the correct destination volume IDs when pools
+  have different IDs across clusters. The RPC supports:
+  - Volume replication: Maps source volume ID to destination volume ID
+  - Volume group replication: Maps source group ID and all member volume
+    IDs to their destination IDs
+  - Pool name-based mapping via `replicationDestination` ConfigMap
+    configuration
+  - Backward compatibility with existing cluster-mapping.json via
+    ClientProfileMapping integration
+
 ## NOTE