vm: update vm-page documentation

include/mango/vm.h

@@ -223,9 +223,13 @@ struct vm_page {
 	/* multi-purpose list/tree entry.
 	   the owner of the page can decide what to do with this.
 	   some examples:
-	     - the buddy allocator uses this to maintain its per-zone free-page lists.
-	     - the block cache uses this to maintain a tree of pages keyed by block number.
-        */
+	     - the buddy allocator uses this to maintain its per-zone free-page
+	       lists.
+	     - vm_object uses it to maintain a btree of allocated pages keyed
+	       by offset/size.
+	     - the block cache uses this to maintain a tree of pages keyed by
+	       block number.
+	*/
 	union {
 		struct queue_entry p_list;
 		struct btree_node p_bnode;