[clang] Mention -Werror changes revived for Clang 16

-Wimplicit-function-declaration and -Wimplicit-int become errors
by default in Clang 16 (originally in 15, but we reverted it in 15.0.1).

Mention it in the release notes like we did originally for Clang 15.

See https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213 for more context.

Signed-off-by: Sam James <sam@gentoo.org>

Differential Revision: https://reviews.llvm.org/D135545

clang/docs/ReleaseNotes.rst

@@ -61,6 +61,17 @@ code bases.
   into an error-only diagnostic in the next Clang release. Fixes
   `Issue 50055 <https://github.com/llvm/llvm-project/issues/50055>`_.
 
+- The ``-Wimplicit-function-declaration`` and ``-Wimplicit-int`` warnings
+  now default to an error in C99, C11, and C17. As of C2x,
+  support for implicit function declarations and implicit int has been removed,
+  and the warning options will have no effect. Specifying ``-Wimplicit-int`` in
+  C89 mode will now issue warnings instead of being a noop.
+
+  **NOTE**: We recommend that projects using configure scripts verify that the
+  results do not change before/after setting
+  ``-Werror=implicit-function-declarations`` or ``-Wimplicit-int`` to avoid
+  incompatibility with Clang 16.
+
 - ``-Wincompatible-function-pointer-types`` now defaults to an error in all C
   language modes. It may be downgraded to a warning with
   ``-Wno-error=incompatible-function-pointer-types`` or disabled entirely with