ocsp: update ParseRequest docs & error conditions

ParseRequest was documented as erroring for inputs that contained
an optional request signature, and as only supporting requests for
a single certificate.

However, prior to this commit DER inputs that contained a signature
succeeded without verifying the signature, and inputs that contained
multiple certificate identifiers succeeded but only the first
certificate identifier was unmarshalled.

This commit updates the function behaviour to return an error for the
signature case as described in the docs, and to clarify the
documentation for inputs with multiple certificate identifiers.

Fixes golang/go#80300

Change-Id: I7446422d6c98bd406b8c43e55fea9a16ace9d4a6
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/798220
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>