Liberalising IncoherentInstances

(Almost) as suggested by SPJ on
http://www.haskell.org/pipermail/glasgow-haskell-users/2013-July/022651.html
(fixes #8141)

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Improve documentation of the new IncoherentInstances behaviour

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Release note blurb about IncoherentInstances.

Signed-off-by: Austin Seipp <aseipp@pobox.com>