This fixes the documentation for -Wall.
As was done previously it leaves out deprecated flags and also
fwarn-safe and fwarn-unsafe. I don't know
if that was intended or not. -fwarn-safe and fwarn-unsafe are
not mentioned on the warnings page at all instead they are
mentioned in the safe haskell section.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D889

GHC Trac Issues: #10386