[KeyInstr] Add release note following #149509 (#156005)

Key Instructions (-gkey-instructions) is now enabled by default when DWARF is
being emitted, the input is plain C/C++, and optimisations are enabled.

Add release note for the change in default behaviour.

Author: OCHyams

clang/docs/ReleaseNotes.rst

@@ -219,6 +219,7 @@ Deprecated Compiler Flags
 
 Modified Compiler Flags
 -----------------------
+- The `-gkey-instructions` compiler flag is now enabled by default when DWARF is emitted for plain C/C++ and optimizations are enabled. (#GH149509)
 
 Removed Compiler Flags
 -------------------------