Update Coercible docs due to Safe Haskell adjustment

This should go with [59722295bb8da8f01d37356fbed6aef7321a8195/ghc], see
bug #8826.

(cherry picked from commit db4f5e5245d5b24a8f0a06a85ded89c6124fb4c7)
Refer to the coercible paper in Coercible' docs

Implements parts of #8888.

(cherry picked from commit fc7aaf57a33ab07b70628c75fcf134fdf4e701e5)