Update the doc for CreateCertificateRequest
to state that it creates a
  `new certificate request`
instead of just a
  `new certificate`

Fixes #14649.

Change-Id: Ibbbcf91d74168998990990e78e5272a6cf294d51
Reviewed-on: https://go-review.googlesource.com/23204Reviewed-by: Russ Cox <rsc@golang.org>