Doc: Hide QTextInlineObject constructor from the documentation

The constructor takes an internal class instance as a parameter,
and is not meant to be used externally. Add a note in the
description and hide the constructor.

Task-number: QTBUG-39785
Change-Id: Ice850f0968880f3933ebb7ae83efeaeed85441cb
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>