Doc: Stop automatic linking to macOS

The link target does not make sense in Qt Creator context.
Explicit linking to \l macOS will still work.

Change-Id: I1a77bf87e889a5a6c374e56a1286c0f39d0585a2
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

doc/src/external-resources/external-resources.qdoc

+/****************************************************************************
+**
+** Copyright (C) 2017 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Free Documentation License Usage
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file. Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*
+    This prevents autolinking of each occurrence of 'macOS'.
+    Explicit links will still work (\l macOS).
+*/
+/*!
+    \externalpage nolink
+    \title macOS
+    \internal
+*/