crypto/x509: document support for leading periods in DNS constraints

crypto/x509: document support for leading periods in DNS constraints

This change adds a comment to the Verify documentation that indicates
that you can use URI and email style name constraints with a leading
period for DNS names (and explains what they do). This behavior is
not standards compliant, but matches the community application of
RFC 5280, so it makes sense to document it.

Fixes #37535

Change-Id: Ibd6f039e4fa46d40ad7ae1ab48eab86f13cf8eff
Reviewed-on: https://go-review.googlesource.com/c/go/+/233525
Reviewed-by: Ryan Sleevi <sleevi@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>