zfs(8) fixes

Documentation fixes for zfs(8)

• White space issue in the userused@user property section
• zfs send supports using bookmarks as the origin snapshot

Reviewed by: Ned Bass <bass6@llnl.gov>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: DHE <git@dehacked.net>
Closes #5906