Docs touch-up for Rejections

[seanmonstar]

With the changes to the Rejection system (#115), the docs could use some improvements.

• The docs for warp::reject::custom should link to Filter::recover.
• The docs for warp::Rejection should link to the warp::reject module documentation.
• The warp::reject module docs should expand on how to recover from rejections.

[boxdot]

Looks like this was already done in the commit b262f93.

[jxs]

The warp::reject module docs should expand on how to recover from rejections.

only this one is missing

[calvinbrown085]

I can grab

The warp::reject module docs should expand on how to recover from rejections.

[calvinbrown085]

Is there a specific example on what kind of rejection you'd want to recover from?

[jxs]

fixed with #876