Document the -X and -T options to 'zpool import'

These options have existed for a long time but have historically
been undocumented because they are not guaranteed to be safe. They
should only be used as a last resort when attempting to recover a
damaged pool.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1130