Update helper documentation
* We've been supplying the CSR in the environment for a long time.
* You can now add test CA configurations with "getcert add-ca".
* We don't bother to set $CERTMONGER_SCEP_NONCE (the bundled helper
doesn't need it), but add $CERTMONGER_SCEP_CERTIFICATES.
* Describe FETCH-SCEP-CA-CAPS and FETCH-SCEP-CA-CERTS operations.
* Describe the "need SCEP data" exit status.
* Note that we now handle multiple types of output data when the helper
manages to get a certificate.