Deny warnings page is out of date

[mvertescher]

The deny warnings page is a little out of date and should be updated to remove warnings that are no longer relevant.

[simonsan]

I wonder if it might be a better approach instead of listing single warnings that could be safe to deny (which will probably change more often in the future as well), to point to rustc -W help and let people pick the warnings they want to deny from that list by themselves. With the mindset: "If you want to deny single warnings please be responsible and pick yourself. We give you a recommendation which allowed lint checks might make sense to deny as well.

Also we could name lint groups provided by rustc rather than single lints.

Thoughts?