Duplicate the encryption copies=3 limitation

This adds the encryption copies=3 limitation language into the copies
property section.
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8651 

man/man8/zfs.8

@@ -1342,6 +1342,11 @@ create, for example a two-disk striped pool and set
 on some datasets thinking you have setup redundancy for them. When a disk
 fails you will not be able to import the pool and will have lost all of your
 data.
+.Pp
+Encrypted datasets may not have
+.Sy copies Ns = Ns Em 3
+since the implementation stores some encryption metadata where the third copy
+would normally be.
 .It Sy devices Ns = Ns Sy on Ns | Ns Sy off
 Controls whether device nodes can be opened on this file system.
 The default value is