Add write_barrier docs

src/gc.c

@@ -65,10 +65,11 @@
 
   mruby implementer and C extension library writer must write a write
   barrier when writing a pointer to an object on object's field.
-  Two different write barrier are available:
+  When writing a pointer to B object on A object's field, 
+  two different write barrier are available:
 
-    * mrb_field_write_barrier
-    * mrb_write_barrier
+    * mrb_field_write_barrier - target B object for a mark.
+    * mrb_write_barrier       - target A object for a mark.
 
   == Generational Mode