Include relevant example code in documentation of setUrlHandler

By only looking at the function prototype, it's tempting to assume that
the handler needs to be registered with the usual SLOT macro. That is
not the case.

The code snippet is already included in the class description, but it
doesn't hurt to repeat it here.

Change-Id: If24fdca41a4bd976ebd1156c9e1106469388265c
Fixes: QTBUG-50484
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

src/gui/util/qdesktopservices.cpp

@@ -244,6 +244,8 @@ bool QDesktopServices::openUrl(const QUrl &url)
     The provided method must be implemented as a slot that only accepts a single QUrl
     argument.
 
+    \snippet code/src_gui_util_qdesktopservices.cpp 0
+
     To use this function for receiving data from other apps on iOS you also need to
     add the custom scheme to the \c CFBundleURLSchemes list in your Info.plist file: