doc: document that weekly.html is only a historical reference
doc: document that weekly.html is only a historical reference

Fixes issue 4810.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7401050

apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang