Update docs of fshl/r

Author: ckoparkar

clang/docs/LanguageExtensions.rst

@@ -875,12 +875,14 @@ of different sizes and signs is forbidden in binary and ternary builtins.
                                                 for the comparison.
 T __builtin_elementwise_fshl(T x, T y, T z)     perform a funnel shift left. Concatenate x and y (x is the most        integer types
                                                 significant bits of the wide value), the combined value is shifted
-                                                left by z, and the most significant bits are extracted to produce
+                                                left by z (modulo the bit width of the original arguments),
+                                                and the most significant bits are extracted to produce
                                                 a result that is the same size as the original arguments.
 
 T __builtin_elementwise_fshr(T x, T y, T z)     perform a funnel shift right. Concatenate x and y (x is the most       integer types
                                                 significant bits of the wide value), the combined value is shifted
-                                                right by z, and the least significant bits are extracted to produce
+                                                right by z (modulo the bit width of the original arguments),
+                                                and the least significant bits are extracted to produce
                                                 a result that is the same size as the original arguments.
  T __builtin_elementwise_ctlz(T x[, T y])       return the number of leading 0 bits in the first argument. If          integer types
                                                 the first argument is 0 and an optional second argument is provided,