Clarify `_url` v `_path` named routes guideline

* Using a `_url` suffix in redirects better matches [RFC 2616 spec, section 14.
  14.30](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.30),
  which states the Location response-header field should use an absolute
  URI for 3xx responses. Rails does this automatically but nice to be
  more intentional.