Replaced IF_NO_GC() macro calls with the more descriptive IF_NO_ARC()

and deprecate it.  Searched for and removed obsolete references to
garbage collection in comments and documentation.

Source/NSClassDescription.m

@@ -75,7 +75,7 @@ static NSMapTable	*classMap;
                         object: aClass];
       description = NSMapGet(classMap, aClass);
     }
-  IF_NO_GC([description retain];)
+  IF_NO_ARC([description retain];)
   [mapLock unlock];
 
   return AUTORELEASE(description);