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

man/man8/zpool-remove.8

@@ -55,6 +55,8 @@ This command supports removing hot spare, cache, log, and both mirrored and
 non-redundant primary top-level vdevs, including dedup and special vdevs.
 When the primary pool storage includes a top-level raidz vdev only hot spare,
 cache, and log devices can be removed.
+Note that keys for all encrypted datasets must be loaded for top-level vdevs
+to be removed.
 .sp
 Removing a top-level vdev reduces the total amount of space in the storage pool.
 The specified device will be evacuated by copying all allocated space from it to