Man zpool-scrub.8: describe sequential scrub

Describe sequential scrub and add examples of scrub status.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #12429