qdoc: support annotations for ownership

As discussed during the Qt for Python documentation session during the Qt Contributors Summit 2019, and since it was pointed out on an email discussion, it would be a good idea to include an annotation that indicates when parameters delegate their ownership to the caller.

An example of a function that doesn't document this detail
https://doc.qt.io/qt-5/qboxlayout.html#addLayout

An example of a function that does document this detail:
https://doc.qt.io/qt-5/qlayout.html#addItem

I guess te same message could be the same from the last one:
Note: The ownership of ____ is transferred to the layout, and it's the layout's responsibility to delete it.

During the email discussion, an alternative tag (like \a) was suggested: \o (or even \ao)

This will help the current process of ownership transfers in the Qt for Python project, from which is currently done by hand, on the XML typesystem