crypto/elliptic: update P-521 docs to say it's constant-time

This is true since CL 315274.

Also adjust the P-256 note, since Add, Double, and IsOnCurve use the
generic, non-constant-time implementation.

Change-Id: I4b3b340f65bce91dcca30bcf86456cc8ce4dd4bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/325650
Trust: Filippo Valsorda <filippo@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/crypto/elliptic/elliptic.go

@@ -455,7 +455,7 @@ func initP384() {
 // Multiple invocations of this function will return the same value, so it can
 // be used for equality checks and switch statements.
 //
-// The cryptographic operations are implemented using constant-time algorithms.
+// ScalarMult and ScalarBaseMult are implemented using constant-time algorithms.
 func P256() Curve {
 	initonce.Do(initAll)
 	return p256
@@ -479,7 +479,7 @@ func P384() Curve {
 // Multiple invocations of this function will return the same value, so it can
 // be used for equality checks and switch statements.
 //
-// The cryptographic operations do not use constant-time algorithms.
+// The cryptographic operations are implemented using constant-time algorithms.
 func P521() Curve {
 	initonce.Do(initAll)
 	return p521