[docs] Add reference to documentation in Attr.def to nudge people.

I noticed that new underscored attributes get added to the end of the file,
so I put the suggestion to add docs at the end instead of the beginning.

Author: varungandhi-apple

include/swift/AST/Attr.def

@@ -670,6 +670,9 @@ SIMPLE_DECL_ATTR(_assemblyVision, EmitAssemblyVisionRemarks,
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
   120)
 
+// If you're adding a new underscored attribute here, please document it in
+// docs/ReferenceGuides/UnderscoredAttributes.md.
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS