Add tiny amount of documentation

collection.h

@@ -17,6 +17,14 @@ struct item_type_vtable
 	int size;
 	void (*free)( struct collection it, void* item );
 	int (*compare)( struct collection it, void* a, void* b );
+	/*
+		Should return:
+		   -1 when a < b
+		    1 when a > b
+		    0 when a = b
+
+		Same as strcmp
+	*/
 };
 
 struct collection