x/crypto/ssh: remove misleading comment, add example

Add an example for using the PublicKeys AuthMethod.

Change-Id: I3fe02bb3c9b8ccf313d72858328c8576cbf3eb06
Reviewed-on: https://go-review.googlesource.com/22250
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
1 file changed