Also adds missing docs.

Change-Id: Ibd8dbe8441bc7a41f01ed2e2033db98e479a5176
Reviewed-on: https://go-review.googlesource.com/40412
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>