vhost-user.rst: Improve [GS]ET_VRING_BASE doc

vhost-user.rst: Improve [GS]ET_VRING_BASE doc

GET_VRING_BASE does not mention that it stops the respective ring.  Fix
that.

Furthermore, it is not fully clear what the "base offset" these
commands' documentation refers to is; an offset could be many things.
Be more precise and verbose about it, especially given that these
commands use different payload structures depending on whether the vring
is split or packed.

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20231016134243.68248-2-hreitz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>