Fix some documentation errors.

Correct links and fix typos, remove obsolete documentation,
fix some snippets, mark some classes as internal.

Change-Id: I9a3266605f060783413d32740057a57a820c8929
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Fix tst_QTextScriptEngine failures on some platforms

The text shaping process has been unified for all platforms.

Task-number: QTBUG-23064
Task-number: QTBUG-24565
Task-number: QTBUG-26495

Change-Id: Ida4c4be85f1a72c04399d2e6760fa36b33a0464e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>