layout: Add more description for layout's internal smart data members.
layout: Add more description for layout's internal smart data members.

tasn/elementary,rvandegrift/elementary,FlorentRevest/Elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,FlorentRevest/Elementary,rvandegrift/elementary,tasn/elementary,FlorentRevest/Elementary,tasn/elementary,rvandegrift/elementary,FlorentRevest/Elementary