godoc: redirect bogus /doc/spec links to /ref/spec
godoc: redirect bogus /doc/spec links to /ref/spec

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://codereview.appspot.com/14348043

t-yuki/go.tools.old,t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools