Doc: Clarify how to use QPainter with QTextEdit

The documentation suggests that it is possible to reimplement
QTextEdit::paintEvent(), but doing so might produce cryptic runtime
warnings:

    QWidget::paintEngine: Should no longer be called
    QPainter::begin: Paint device returned engine == 0, type: 1

The correct way to reimplement this function is noted under
QAbstractScrollArea::paintEvent(). This patch updates the note and
copies it to QTextEdit for clarity.

Change-Id: Ib7d8dadeb2358475bcdb0b2e624857700f9a004e
Reviewed-by: Safiyyah Moosa <safiyyah.moosa@qt.io>
Reviewed-by: Andreas Eliasson <andreas.eliasson@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 5a2474b2dd0472948c53b16c4bf566db489dcfb9)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qabstractscrollarea.cpp

@@ -1100,7 +1100,7 @@ void QAbstractScrollArea::resizeEvent(QResizeEvent *)
     This event handler can be reimplemented in a subclass to receive
     paint events (passed in \a event), for the viewport() widget.
 
-    \note If you open a painter, make sure to open it on the viewport().
+    \note If you create a QPainter, it must operate on the viewport().
 
     \sa QWidget::paintEvent()
 */

src/widgets/widgets/qtextedit.cpp

@@ -1599,6 +1599,8 @@ void QTextEditPrivate::paint(QPainter *p, QPaintEvent *e)
 This event handler can be reimplemented in a subclass to receive paint events passed in \a event.
 It is usually unnecessary to reimplement this function in a subclass of QTextEdit.
 
+\note If you create a QPainter, it must operate on the \l{QAbstractScrollArea::}{viewport()}.
+
 \warning The underlying text document must not be modified from within a reimplementation
 of this function.
 */