QLabel documentation: add warning about sanitizing input

make users more aware of QLabel guessing the text format.

Reviewed-by: Gabriel de Dietrich <gabriel.dietrich-de@nokia.com>
(cherry picked from commit 31f7ecbdcdbafbac5bbfa693e4d060757244941b)

Change-Id: I681c7daf568a6107b01908fa85d2da2a7e4e228a
Reviewed-on: http://codereview.qt.nokia.com/2817
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Peter Hartmann <peter.hartmann@nokia.com>

src/gui/widgets/qlabel.cpp

@@ -88,6 +88,13 @@ QT_BEGIN_NAMESPACE
             by clear().
     \endtable
 
+    \warning When passing a QString to the constructor or calling setText(),
+    make sure to sanitize your input, as QLabel tries to guess whether it
+    displays the text as plain text or as rich text. You may want to call
+    setTextFormat() explicitly, e.g. in case you expect the text to be in
+    plain format but cannot control the text source (for instance when
+    displaying data loaded from the Web).
+
     When the content is changed using any of these functions, any
     previous content is cleared.