mount_unionfs: remove jokey cautions from man page

There are known issues with unionfs, and the mount_unionfs man page has
a cautionary statement about its use. The caution had additional
"humourous" statements like "BEWARE OF DOG" but they served only to
confuse the situation. Remove them.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 6659516b1a47e34c184c62b8ad7982e5466db539)