Clarify behavior of DuplicateProperty linter

There was some confusion about how smart this linter was at detecting
intentional duplicate properties. Add documentation emphasizing that the
check is dumb.

Change-Id: Ib29e0b3ac379e9a9e54012abc6cc5df87b45df03
Reviewed-on: http://gerrit.causes.com/36947
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>