Mention QRect's int min/max constraints in detailed description.

Task-number: QTBUG-25732

Change-Id: If330768c3075568f09593ed17f26389d3dec3335
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/corelib/tools/qrect.cpp

@@ -201,6 +201,12 @@ QT_BEGIN_NAMESPACE
     function to manipulate the rectangle's coordinates and dimensions
     in one go.
 
+    \section1 Constraints
+
+    QRect is limited to the minimum and maximum values for the \c int type.
+    Operations on a QRect that could potentially result in values outside this
+    range will result in undefined behavior.
+
     \sa QRectF, QRegion
 */