Doc: Add a comment into config.qdocconf about generating internal docs

QDoc supports the --showinternal command line option, but it's
not integrated to the build system.

This commit documents an alternative way to get documentation marked as
internal generated, replacing every instance of \internal with a text
string '[internal]'

Change-Id: Ib48ade21adf32d430e62b95c4baa64be620d7eb0
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>

doc/global/config.qdocconf

@@ -13,3 +13,7 @@ navigation.homepage = "Qt $QT_VER"
 sourcedirs += includes $$BUILDDIR
 
 url = http://doc.qt.io/qt-5
+
+# Uncomment the following two lines to generate documentation marked as \internal
+# alias.internal = disable
+# macro.internal.HTML = "<span style="color:red">[internal]</span>"