Warn user about accidentally sharing devices

Warn user about accidentally sharing devices

Improve the man page text to warn the user about the risk of adding
the same device to multiple pools via simultaneous "zpool create",
"zpool add", "zpool replace", etc.

State that MMP/multihost does not protect against these scenarios.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #6473
Closes #8457