language: move package comment to doc.go Change-Id: Idf694a566e3e3ccc1ea0a3780c55468d237db463 Reviewed-on: https://go-review.googlesource.com/59813 Run-TryBot: Marcel van Lohuizen <mpvl@golang.org> Reviewed-by: Nigel Tao <nigeltao@golang.org>