LoadX509KeyPair and X509KeyPair don't retain the parsed form of
certificates in their return value because it's generally not needed.
This change makes that clear in the comment.

See https://groups.google.com/d/msg/golang-dev/VResvFj2vF8/Wt6WkVT2AwAJ

Change-Id: Ibb759cd6e84c00f4450a012992088422c0546638
Reviewed-on: https://go-review.googlesource.com/18734Reviewed-by: Russ Cox <rsc@golang.org>