Doc: fix QStandardPaths::displayName qdoc warning on macOS

The function definition at the location of the documentation is not used
on Q_OS_DARWIN, so qdoc cannot tie the documentation to it, leaving
broken links and qdoc warnings when building the documentation on
macOS.

Fix this by explicitly specifying the function that's being documented.

Pick-to: 6.7
Change-Id: Icf2ad1eba9e9ec8cbeea3818b414f740fa31083f
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Safiyyah Moosa <safiyyah.moosa@qt.io>

src/corelib/io/qstandardpaths.cpp

@@ -540,6 +540,8 @@ QString QStandardPaths::findExecutable(const QString &executableName, const QStr
 }
 
 /*!
+    \fn QString QStandardPaths::displayName(StandardLocation type)
+
     \include standardpath/functiondocs.qdocinc displayName
 */