doc: running on different seat with libseat

When using the libseat launcher, there is one more detail to take care:
stop libseat from managing the VT.

A normal user does not have permissions to manage a VT, so launching
would just fail. In this use case you also do not want to be managing
the VT, because your normal desktop is already owning the seat
associated with the VT.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>