Link to RFC about fragments in redirects

This describes that the `Location` header can omit the fragment and the
browser should re-apply it to the new URL.
I think 'browser' is fine also (I don't think we need to account for [fridges](http://tools.ietf.org/html/rfc7230#section-2.2) just yet), but my point was that we don't currently say _what_ should reapply them (it reads 'because should' currently).