docs: remove incorrect 'size' from output of 'snapshot info'

docs: remove incorrect 'size' from output of 'snapshot info'

The 'size' shown in the output of snapshot info command relies on
rstats which is incorrect snapshot size. It tracks size of the
subvolume from the snapshot has been taken instead of the snapshot
itself. Hence having the 'size' field in the output of 'snapshot info'
doesn't make sense until the rstats is fixed.

Fixes: https://tracker.ceph.com/issues/55822
Signed-off-by: Nikhilkumar Shelke <nshelke@redhat.com>