Make relation of currentChildBeingDeleted and isDeletingChildren more obvious

Add a comment, to describe the relation of
QObjectPrivate::currentChildBeingDeleted and
QObjectData::isDeletingChildren, given that this field is defined
in an entirely other file on a base-class, making it severely
non-obvious to the reader.

Task-number: QTBUG-57714
Change-Id: I7b8b24693c4ccc6192e9f9f34f951202b7fdddb0
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/kernel/qobject_p.h

@@ -234,7 +234,7 @@ public:
     mutable quint32 connectedSignals[2];
 
     union {
-        QObject *currentChildBeingDeleted;
+        QObject *currentChildBeingDeleted; // should only be used when QObjectData::isDeletingChildren is set
         QAbstractDeclarativeData *declarativeData; //extra data used by the declarative module
     };