go/build: document the go1.2 build tag
go/build: document the go1.2 build tag

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/14930046

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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