zpool-remove.8: describe top-level vdev sector size limitation

Document that top-level vdevs cannot be removed unless all top-level
vdevs have the same sector size.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Sam Hathaway <sam@sam-hathaway.com>
Closes #11339
Closes #12472