image: add available godoc link

image: add available godoc link

Change-Id: I2839ecb091c4f0b30d0dcee708bf9e9a55e3672a
Reviewed-on: https://go-review.googlesource.com/c/go/+/535196
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: shuang cui <imcusg@gmail.com>