update docs

Author: frasercrmck

clang/docs/LanguageExtensions.rst

@@ -649,7 +649,9 @@ Unless specified otherwise operation(±0) = ±0 and operation(±infinity) = ±in
 
 The integer elementwise intrinsics, including ``__builtin_elementwise_popcount``,
 ``__builtin_elementwise_bitreverse``, ``__builtin_elementwise_add_sat``,
-``__builtin_elementwise_sub_sat`` can be called in a ``constexpr`` context.
+``__builtin_elementwise_sub_sat`` can be called in a ``constexpr`` context. No
+implicit promotion of integer types takes place. The mixing of integer types of
+different sizes and signs is forbidden in binary and ternary builtins.
 
 ============================================== ====================================================================== =========================================
          Name                                   Operation                                                             Supported element types