Improve seq documentation; part of trac issue #9390

Summary: Signed-off-by: Michael Snoyman <michael@snoyman.com>

Test Plan: Review documentation change

Reviewers: simonpj, austin

Reviewed By: austin

Subscribers: phaskell, hvr, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D136

GHC Trac Issues: #9390