AvailabilityAttr: update the document for macosx

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274133 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/AttrDocs.td

@@ -746,6 +746,8 @@ are:
 ``macos``
   Apple's Mac OS X operating system.  The minimum deployment target is
   specified by the ``-mmacosx-version-min=*version*`` command-line argument.
+  ``macosx`` is supported for backward-compatibility reasons, but it is
+  deprecated.
 
 ``tvos``
   Apple's tvOS operating system.  The minimum deployment target is specified by