write barrier doc update

src/gc.c

@@ -63,10 +63,10 @@
 
   == Write Barrier
 
-  mruby implementer and C extension library writer must write a write
-  barrier when writing a pointer to an object on object's field.
-  When writing a pointer to B object on A object's field, 
-  two different write barrier are available:
+  mruby implementer and C extension library writer must insert a write
+  barrier when updating a reference from a field of an object.
+  When updating a reference from a field of object A to object B, 
+  two different types of write barrier are available:
 
     * mrb_field_write_barrier - target B object for a mark.
     * mrb_write_barrier       - target A object for a mark.