docs:monitor: Fill in trust group `group` attribute details

The `group` attribute within 'sensor_trust_groups' contains a list of
fan tach sensors to cancel/resume monitoring based on the trust
determination across all members of the group.

Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
Change-Id: Ifab450c05b7b8cde234d60b58eff1029ce66f5bb
1 file changed