Clarify failmode=wait documentation

Nowhere in the description of the failmode property does it
clearly state how to bring a suspended pool back online.
Add a few words to property description and the zpool-clear(8)
man page.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12907
Closes #9395