folly/io/async documentation

Summary: Documentation brain-dump.  The sources already have pretty good doxygen-style comments, but could use a good high-level overview.

Test Plan: Reviewers get to edit

Reviewed By: jsedgwick@fb.com, afrind@fb.com, dcsommer@fb.com, alandau@fb.com

Subscribers: fugalh, subodh, kmdent, seanc, vloh, sjenkins, kparton, doug, net-systems@, njormrod, folly-diffs@, sgolemon, alikhtarov, andrii, tudorb

FB internal diff: D1690976

Tasks: 5648428

Signature: t1:1690976:1416419706:3894976c32904f040d518b9a4ce26ccb40d7ed80