unit-test: Add some doc on Association
unit-test: Add some doc on Association

Next set of unit tests use this data structure so added some doc and an
example

Change-Id: Iabc89fc66ff68ccbf872e8cb48dc75b75c78908f
Signed-off-by: Andrew Geissler <f7919ac9f8ec0c3ffbc3f483da625c634fd31638@yahoo.com>

openbmc/phosphor-objmgr,openbmc/phosphor-objmgr