QComboBox: mention insert policy in context of editable property

This makes it a little bit easier for the reader to discover what
editable combo boxes are about.

Change-Id: I60ce571f01e32d4cacb4718b42a8e12884bd1c13
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/widgets/widgets/qcombobox.cpp

@@ -1597,7 +1597,10 @@ void QComboBox::setIconSize(const QSize &size)
     \property QComboBox::editable
     \brief whether the combo box can be edited by the user
 
-    By default, this property is false.
+    By default, this property is false. The effect of editing depends
+    on the insert policy.
+
+    \sa InsertPolicy
 */
 bool QComboBox::isEditable() const
 {