RELEASE_NOTES.md: AgX warning for HLG target white

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018Hf5bYk6AYsiHXNmKFEr15

Author: claude

RELEASE_NOTES.md

@@ -249,7 +249,8 @@ changes (where available).
   reference white, so 1000 nit highlights need about 493%. The curve
   graph rescales to show the extended shoulder. A new "show target
   white in nits" option displays and accepts the value in nits
-  instead.
+  instead. A warning appears when the value exceeds what an HLG export
+  can hold (about 493%), since HLG clips above 1000 nits.
 
 - The external raster masks module imports its mask into the image's
   `.dtdata` sidecar instead of referencing a file in a root folder.