It was not enough to make /ref/spec work (which it now does),
because some of the docs on golang.org (in particular golang.org/doc)
are pulled from tip, and tip links to /doc/spec now.
Make those "too new" links work by redirecting /doc/spec
back to /ref/spec for now.
TBR=adg
CC=golang-dev
https://golang.org/cl/14348043