doc: move general object description as SECTION

Best to have the general overview of the object at the top of the doc
file.

Signed-off-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Reviewed-by: Mathieu Duponchelle <mathieu.duponchelle@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4293