x/crypto/ssh/agent: add a client example and tweak package doc.

Change-Id: I373fdbb6351d71b12fcfed31cf4b08975a443294
Reviewed-on: https://go-review.googlesource.com/19894
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed