Note that keys must be loaded for 'zpool remove'

The error returned by zpool remove when the encryption keys aren't
loaded isn't very helpful. Furthermore, the man pages make no
mention that the keys need to be loaded. This change doesn't resolve
the error message but it does update the man page to mention this
requirement.

Authored-by: grodik <pat@litke.dev>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10939
Closes #10948