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

d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,skyportsystems/go.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools,8limbs/gotools,skyportsystems/go.tools,skyportsystems/go.tools,webfd/go-zh.tools