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

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>