In Go 1.5, Config.Certificates is no longer required if
Config.GetCertificate has been set. This change updated four comments to
reflect that.

Change-Id: Id72cc22fc79e931b2d645a7c3960c3241042762c
Reviewed-on: https://go-review.googlesource.com/13800Reviewed-by: Adam Langley <agl@golang.org>