crypto/tls: note in comment that Certificate.Leaf is nil after parsing.

crypto/tls: note in comment that Certificate.Leaf is nil after parsing.

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/18734
Reviewed-by: Russ Cox <rsc@golang.org>