mount.8: mention that the snapshot option is exclusive

When using the snapshot option, all other options are ignored. This
update reflects changes made in ab2dbd9b871d.

Reviewed by: 0mp, mckusick
Differential Revision: https://reviews.freebsd.org/D34584

(cherry picked from commit 8772a9117b27f551f6fbaae9413acaa30d99f934)