Doc fix in return value of QDate::setYMD

Change-Id: Ie107e451a7b2a084fb6131939f212acd412a9df6
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>

src/corelib/tools/qdatetime.cpp

@@ -847,6 +847,7 @@ QString QDate::toString(const QString& format) const
 
     If \a y is in the range 0 to 99, it is interpreted as 1900 to
     1999.
+    Returns \c false if the date is invalid.
 
     Use setDate() instead.
 */