Explained rectangle bounding in documentation.

Task-number: QTBUG-26756
Change-Id: Ifa56cbe249164e239f21c00fc31cd6ddd81705ad
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>

src/gui/painting/qpainter.cpp

@@ -5928,6 +5928,7 @@ void QPainter::drawText(const QRect &r, int flags, const QString &str, QRect *br
     \overload
 
     Draws the given \a text within the provided \a rectangle.
+    The \a rectangle along with alignment \a flags defines the anchors for the \a text.
 
     \table 100%
     \row