Doc: add notes about validator and completer in QComboBox

When setting the editable property to false, we delete the internal
QLineEdit used by QComboBox. Add comments that this results in the loss
of validator and completer.

Task-number: QTBUG-56035
Change-Id: Ife04ac2b9bb3f32fae5328f1ec73b1d5d769d52c
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>
Reviewed-by: Nico Vertriest <nico.vertriest@theqtcompany.com>

src/widgets/widgets/qcombobox.cpp

@@ -1678,6 +1678,9 @@ void QComboBox::setIconSize(const QSize &size)
     By default, this property is \c false. The effect of editing depends
     on the insert policy.
 
+    \note When disabling the \a editable state, the validator and
+    completer are removed.
+
     \sa InsertPolicy
 */
 bool QComboBox::isEditable() const
@@ -1829,6 +1832,8 @@ QLineEdit *QComboBox::lineEdit() const
     \fn void QComboBox::setValidator(const QValidator *validator)
 
     Sets the \a validator to use instead of the current validator.
+
+    \note The validator is removed when the editable property becomes \c false.
 */
 
 void QComboBox::setValidator(const QValidator *v)
@@ -1862,6 +1867,8 @@ const QValidator *QComboBox::validator() const
 
     By default, for an editable combo box, a QCompleter that
     performs case insensitive inline completion is automatically created.
+
+    \note The completer is removed when the \a editable property becomes \c false.
 */
 void QComboBox::setCompleter(QCompleter *c)
 {