Clearer wording on Errata #4

Clearer wording on Errata #4

Users of existing pools, especially pools with top-level encrypted
datasets, could run into trouble trying to work around Errata #4.
Clarify that removing encrypted snapshots and bookmarks is enough
to clear the errata.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Justin Scholz <git@justinscholz.de>
Closes #8682
Closes #8683