release/20.x: [Clang][ReleaseNotes] Document -fclang-abi-compat=19 re: #110503 (#125368)

#110503 updates the scope of `-fclang-abi-compat` but did not make that
clear in the release notes. This PR addresses that problem.

Author: hubert-reinterpretcast

clang/docs/ReleaseNotes.rst

@@ -252,7 +252,7 @@ ABI Changes in This Version
 
 - Fixed Microsoft name mangling of placeholder, auto and decltype(auto), return types for MSVC 1920+. This change resolves incompatibilities with code compiled by MSVC 1920+ but will introduce incompatibilities with code compiled by earlier versions of Clang unless such code is built with the compiler option -fms-compatibility-version=19.14 to imitate the MSVC 1914 mangling behavior.
 - Fixed the Itanium mangling of the construction vtable name. This change will introduce incompatibilities with code compiled by Clang 19 and earlier versions, unless the -fclang-abi-compat=19 option is used. (#GH108015)
-- Mangle member-like friend function templates as members of the enclosing class. (#GH110247, #GH110503)
+- Mangle member-like friend function templates as members of the enclosing class. This can be disabled using -fclang-abi-compat=19. (#GH110247, #GH110503)
 
 AST Dumping Potentially Breaking Changes
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