docs:monitor: Fill in trust group class attribute details

The `class` attribute within 'sensor_trust_groups' contains the trust
group determination class implementation to use in determining trust of
the fan sensors within the trust group.

Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
Change-Id: I52721974a8af61f2bb912751a016dfcd28d824e6
1 file changed