add doc

Author: c8ef

clang/docs/LanguageExtensions.rst

@@ -732,6 +732,10 @@ at the end to the next power of 2.
 
 These reductions support both fixed-sized and scalable vector types.
 
+The reduction intrinsics, including ``__builtin_reduce_add``,
+``__builtin_reduce_mul``, ``__builtin_reduce_and``, ``__builtin_reduce_or``,
+and ``__builtin_reduce_xor``, can be called in a ``constexpr`` context.
+
 Example:
 
 .. code-block:: c++