interface_scrollable: Add more description to animators.
interface_scrollable: Add more description to animators.

It is surprising that there is no such a comment on a scrollerable
interface considering the complexity of scroller.

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