[clang] [doc] Document that the ms_abi attribute works on aarch64 too (#166373)

Since 022e782 (2017) this attribute has
an effect on both aarch64 and x86_64; update the docs to reflect this.

Author: mstorsjo

clang/include/clang/Basic/AttrDocs.td

@@ -3450,9 +3450,9 @@ Mac, and BSD. This attribute has no effect on other targets.
 def MSABIDocs : Documentation {
   let Category = DocCatCallingConvs;
   let Content = [{
-On non-Windows x86_64 targets, this attribute changes the calling convention of
-a function to match the default convention used on Windows x86_64. This
-attribute has no effect on Windows targets or non-x86_64 targets.
+On non-Windows x86_64 and aarch64 targets, this attribute changes the calling convention of
+a function to match the default convention used on Windows. This
+attribute has no effect on Windows targets or non-x86_64, non-aarch64 targets.
   }];
 }