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 <geissonator@yahoo.com>
diff --git a/src/associations.hpp b/src/associations.hpp
index 7cd3842..91a9fcd 100644
--- a/src/associations.hpp
+++ b/src/associations.hpp
@@ -45,6 +45,9 @@
 using AssociationOwnersType = boost::container::flat_map<
     std::string, boost::container::flat_map<std::string, AssociationPaths>>;
 
+// Store the contents of the associations property on the interface
+// For example:
+// ["inventory", "activation", "/xyz/openbmc_project/inventory/system/chassis"]
 using Association = std::tuple<std::string, std::string, std::string>;
 
 /** @brief Remove input association