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

src/crypto/tls/tls.go

@@ -172,7 +172,9 @@ func Dial(network, addr string, config *Config) (*Conn, error) {
 }
 
 // LoadX509KeyPair reads and parses a public/private key pair from a pair of
-// files. The files must contain PEM encoded data.
+// files. The files must contain PEM encoded data. On successful return,
+// Certificate.Leaf will be nil because the parsed form of the certificate is
+// not retained.
 func LoadX509KeyPair(certFile, keyFile string) (Certificate, error) {
 	certPEMBlock, err := ioutil.ReadFile(certFile)
 	if err != nil {
@@ -186,7 +188,8 @@ func LoadX509KeyPair(certFile, keyFile string) (Certificate, error) {
 }
 
 // X509KeyPair parses a public/private key pair from a pair of
-// PEM encoded data.
+// PEM encoded data. On successful return, Certificate.Leaf will be nil because
+// the parsed form of the certificate is not retained.
 func X509KeyPair(certPEMBlock, keyPEMBlock []byte) (Certificate, error) {
 	fail := func(err error) (Certificate, error) { return Certificate{}, err }