scope.h: Add comment

This is a word to the wise to avoid what the previous commit fixed:
changing something here, but not updating the corresponding
documentation which is located elsewhere.

Author: khwilliamson

scope.h

@@ -157,6 +157,7 @@ scope has the given name. C<name> must be a literal string.
         if (PL_savestack_ix > old) leave_scope(old); \
     } STMT_END
 
+/* N.B. These are documented in pad.h */
 #define SAVEI8(i)                   save_I8((I8*)&(i))
 #define SAVEI16(i)                  save_I16((I16*)&(i))
 #define SAVEI32(i)                  save_I32((I32*)&(i))