Link this_fiber::thread_affinity() to fiber::thread_affinity().

Use [#named_anchor] QuickBook functionality rather than trying to predict the
normalized link for [heading Member function `thread_affinity( bool)`], which
looks like this:

fiber.fiber_mgmt.fiber.member_function__code__phrase_role__identifier__thread_affinity__phrase__phrase_role__special_____phrase___phrase_role__keyword__bool__phrase__phrase_role__special_____phrase___code_