QTest::ignoreMessage: Clarify what encoding we assume for parameter

So future-me and others don't have to look through the source code to
figure it out.

Pick-to: 6.8 6.5
Change-Id: I1f6e46cec256112226cad4d24ed52e4818635ae8
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit d2d74673f3dccc04894f2ea67fc7662cc6ef400b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/testlib/qtestcase.cpp

@@ -2187,6 +2187,8 @@ void QTest::qWarn(const char *message, const char *file, int line)
 
     The example above tests that QDir::mkdir() outputs the right warning when invoked
     with an invalid file name.
+
+    \note \a message will be interpreted as UTF-8.
 */
 void QTest::ignoreMessage(QtMsgType type, const char *message)
 {