qapi: Replace ad hoc "since" documentation by member documentation

MemoryDeviceInfoKind, NetClientDriver, and GuestPanicAction mention
some members only in ad hoc since documentation.  The generated
documentation shows these members as "Not documented".

Replace by formal member documentation.

Add actual documentation text for the GuestPanicAction members, to
match existing member documentation there.  For the others, merely
move existing "since" information.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230425064223.820979-14-armbru@redhat.com>