Add release note for explicit(bool) assertion fix

Author: Jongmyeong Choi

clang/docs/ReleaseNotes.rst

@@ -194,6 +194,7 @@ Bug Fixes to C++ Support
 - Fix the dynamic_cast to final class optimization to correctly handle
   casts that are guaranteed to fail (#GH137518).
 - Fix bug rejecting partial specialization of variable templates with auto NTTPs (#GH118190).
+- Fix a crash when using ``explicit(bool)`` in pre-C++11 language modes. (#GH152729)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^