asio: override & final virtual methods, classes

Reviewed By: @jano

Differential Revision: D1582474
Update docblock in hh_shared

Summary: The previous version was misleading at best (I'd probably say
"full of lies"). This corrects some of the misleading bits and
elaborates some more on the details and invariants.

Reviewed By: @int3

Differential Revision: D1604674