Clarify seq documentation

Authored by psibi on Sep 13 2017, 11:33 AM.

Description

Clarify seq documentation

Improves the documentation by specifying that the first argument in
seq function is evaluated to WHNF.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: harendra, rwbarton, thomie

GHC Trac Issues: Trac #14213

Differential Revision: https://phabricator.haskell.org/D3945