Update zpool(8) to clarify what type of vdevs may be safely
removed and that the existence of any top-level raidz device
which is part of the primary pool will prevent device removal.
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#7880Closes#7893