crypto/x509: document fields used in CreateCertificate
crypto/x509: document fields used in CreateCertificate

The added fields are used in buildExtensions so
should be documented too.

Fixes #21363

Change-Id: Ifcc11da5b690327946c2488bcf4c79c60175a339
Reviewed-on: https://go-review.googlesource.com/113916
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

akutz/go,golang/go,nawawi/go,josharian/go,CAFxX/go,nawawi/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,josharian/go,golang/go,akutz/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,josharian/go,golang/go,josharian/go,golang/go,josharian/go,akutz/go,nawawi/go,golang/go,josharian/go,akutz/go,golang/go,nawawi/go,cloudflare/go,josharian/go,cloudflare/go,CAFxX/go,josharian/go,cloudflare/go