Clarify and improve encryption documentation

• Remove the language that "all user data" is encrypted. This is to avoid misunderstandings or arguments about what is "user data", especially in light of "user properties".
• Document that properties are unencrypted.
• Document that snapshot names are unencrypted.
• For consistency with the rest of the zfs.8 man page, use "ZFS" as the generic noun, not (bolded) "zfs". The latter refers to the command. Likewise, use "ZFS" instead of "the kernel module".
• Give "a passphrase" as an example of a "user's key".

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8652