documentation comment (small correction about strings being

gray)

lgc.h

@@ -1,5 +1,5 @@
 /*
-** $Id: lgc.h,v 2.51 2011/09/30 12:44:19 roberto Exp roberto $
+** $Id: lgc.h,v 2.52 2011/10/03 17:54:25 roberto Exp roberto $
 ** Garbage Collector
 ** See Copyright Notice in lua.h
 */
@@ -20,8 +20,10 @@
 ** is that a black object can never point to a white one. Moreover,
 ** any gray object must be in a "gray list" (gray, grayagain, weak,
 ** allweak, ephemeron) so that it can be visited again before finishing
-** the collection cycle. These lists have no meaning when the invariant
-** is not being enforced (e.g., sweep phase).
+** the collection cycle. (These rule does not apply to strings,
+** which are never black but do not need to be visited again.)
+**  These lists have no meaning when the invariant is not being enforced
+** (e.g., sweep phase).
 */