PEL: Add additional HostNotifier documentation
PEL: Add additional HostNotifier documentation

Give a high level overview of how the class works.

Signed-off-by: Matt Spinler <c000c989d08c98662d98f15448f11de169feb92d@us.ibm.com>
Change-Id: I83957e572239997cde6757abb77481890cf98b7a

openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging