More doc.

limmutable.c

@@ -20,6 +20,7 @@
 *     __buckets: weak-keyed map from instance userdata -> hash bucket (used to manage hash bucket lifetimes)
 *     __fields: list of named fields
 *     __indices: map from field names to the index in the instance table where the value is stored
+*     metamethods, methods, class variables, etc.
 */
 
 #include "lua.h"