Improve documentation for Qt::ItemNeverHasChildren.

Change-Id: Ied817314d6e72f08a81138bd817c8570d586fa30
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/global/qnamespace.qdoc

@@ -2562,6 +2562,7 @@
     \value ItemIsEnabled The user can interact with the item.
     \value ItemIsTristate The item is checkable with three separate states.
     \value ItemNeverHasChildren The item never has child items.
+           This is used for optimization purposes only.
 
     Note that checkable items need to be given both a suitable set of flags
     and an initial state, indicating whether the item is checked or not.