go/zodb: Require PModify to be called before actually changing an object

This will allow persistency layer to do kind of copy-on-write, if e.g.
there are many connections that are sharing object's state.

go/zodb/persistent_api.go

@@ -88,9 +88,11 @@ type IPersistent interface {
 
 	// PModify marks in-RAM object state as modified.
 	//
-	// It informs persistency layer that object's data was changed and so
-	// its state needs to be either saved back into database on transaction
-	// commit, or discarded on transaction abort.
+	// It informs persistency layer that object's data is going to be
+	// changed and so its state will need to be either saved back into
+	// database on transaction commit, or discarded on transaction abort.
+	//
+	// PModify must be called before object data is modified.
 	//
 	// The object must be already activated.
 	//PModify()	TODO

go/zodb/zodb.go

@@ -82,8 +82,8 @@
 //
 //	obj.xxx // use object.
 //	if ... {
-//		obj.xxx++     // change object.
-//		obj.PModify() // let persistency layer know we modified the object.
+//		obj.PModify() // let persistency layer know we are going to modifiy the object.
+//		obj.xxx++     // change the object.
 //	}
 //
 //	// tell persistency layer we no longer need obj's in-RAM data to be present.