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
1 file changed