Improve the inline descriptions of the ARC module parameters

These are displayed as the descriptions of the sysctl's on FreeBSD

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #13334