[clang] add release note for constexpr step limit opt-out

Author: Tsche

clang/docs/ReleaseNotes.rst

@@ -258,6 +258,7 @@ 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)
+- The `-fconstexpr-steps` compiler flag now accepts value `0` to opt out of this limit. (#GH160440)
 
 Removed Compiler Flags
 -------------------------