[clang][docs] Fix supported element types for __builtin_reduce_(add|mul)

The docs mention that `__builtin_reduce_add` and `__builtin_reduce_mul` support
both integer and floating point element types, but only integer element types
are actually supported. See https://github.com/llvm/llvm-project/issues/57847,
and specifically,
https://github.com/llvm/llvm-project/blob/00874c48ea4d291908517afaab50d1dcbfb016c3/clang/lib/Sema/SemaChecking.cpp#L2631 for the fact that floating point element types are not supported yet.

Fix the docs to only mention support for integer element types.