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>
2 files changed