quchen authored Dec 16, 2015 and Herbert Valerio Riedel committed Dec 16, 2015

This also updates the user's guide to refer to the `-W`-based warning
flags by default.

Quoting the release note entry:

| Warnings can now be controlled with `-W(no-)...` flags in addition to
| the old `-f(no-)warn...` ones. This was done as the first part of a
| rewrite of the warning system to provide better control over warnings,
| better warning messages, and more common syntax compared to other
| compilers. The old `-fwarn...`-based warning flags will remain
| functional for the forseeable future.

This is part of
https://ghc.haskell.org/wiki/Design/Warnings
and addresses #11218

Reviewed By: hvr, bgamari

Differential Revision: https://phabricator.haskell.org/D1613