Add explicit note about removeMappings() not disconnecting the signal

There can be a bit of confusion here regarding what removeMappings()
will do in this case so add an explicit note to be clearer.

Task-number: QTBUG-49499
Change-Id: Iabcf5cb2653f0b747727b2c92a244e95ec1836f8
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>

src/corelib/kernel/qsignalmapper.cpp

@@ -220,6 +220,9 @@ QObject *QSignalMapper::mapping(QObject *object) const
     Removes all mappings for \a sender.
 
     This is done automatically when mapped objects are destroyed.
+
+    \note This does not disconnect any signals. If \a sender is not destroyed
+    then this will need to be done explicitly if required.
 */
 void QSignalMapper::removeMappings(QObject *sender)
 {