[2.7] Add RST source comment marking PR NVIDIA#4209 dependency in release notes

Add a visible source comment above the Hierarchical FL Startup Stability
section noting that its content depends on PR NVIDIA#4209 and should not be
merged before that PR lands on 2.7.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chesterxgchen

docs/release_notes/flare_272.rst

@@ -268,6 +268,9 @@ outside the lock, with a compare-and-store pattern for the cache write. This sig
 reduces model-download latency when many clients (e.g., 24 per relay) request the same
 chunk concurrently.
 
+.. Changes in this section are introduced by PR #4209 (https://github.com/NVIDIA/NVFlare/pull/4209).
+.. Merge this release notes PR only after PR #4209 has landed on the 2.7 branch.
+
 Hierarchical FL Startup Stability
 -----------------------------------