Mark qmlRegisterCustomExtendedType as \internal

The function was added in 5.4, but it makes use of QQmlCustomParser,
which at the moment is private API. This way Qt3D or bleeding edge users
can still use the function, while keeping freedom to change
QQmlCustomParser.

Since 5.4.0 had the typo in the \relates, users won't see a change from
5.4.0 to 5.4.1.

Change-Id: I7f791f5b39e2a2c9fc83f4c450e6da9eec5b5e46
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>