Document the fact that SignCert sets the Nonce field.  This makes it
clear that callers need not set it themselves.

Original change by Jakob Weisblat <jakobw@mit.edu> in CL 57051.

Fixes golang/go#21384

Change-Id: I161e0909ba6c8d07be5f8e68dc6e0f0392bea63f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/230212
Reviewed-by: Filippo Valsorda <filippo@golang.org>