Help: Clarify 4.2 release note on `$<CONFIG>` policy CMP0199

Clarify the release note added by commit 8ac826a5f2 (GenEx: Fix
evaluation of $<CONFIG> on imported targets, 2025-07-30,
v4.2.0-rc1~439^2) to more precisely describe the behavior change.

See: https://discourse.cmake.org/t/15251/2

Help/release/4.2.rst

@@ -202,8 +202,9 @@ Deprecated and Removed Features
 Other Changes
 =============
 
-* The :genex:`$<CONFIG:cfgs>` generator expression no longer matches multiple
-  configurations.  See policy :policy:`CMP0199`.
+* The :genex:`$<CONFIG:cfgs>` generator expression, when appearing on an
+  imported target, has been fixed to only match the configuration actually
+  being consumed.  See policy :policy:`CMP0199`.
 
 * Selection of configuration and location of imported targets is now more
   consistent.  See policy :policy:`CMP0200`.