Improve documentation of zpool import -d/-c vs -s

Specify that, by default, zpool import uses the libblkid
cache on Linux and geom on FreeBSD, and only scans when
-d/-s is provided.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Signed-off-by: Chris McDonough <chrism@plope.com>
Closes #7656
Closes #10771