(maint) Correct Certificate Factory documentation

The existing certificate factory documentation incorrectly indicated
that it was responsible for actually signing certificates, which it is
not. In addition it specified a parameter as a raw
OpenSSL::X509::Request which it is actually the wrapped
Puppet::SSL::CertificateRequest.
Merge pull request #3151 from adrienthebo/doc/master/certificate-factory

(maint) Correct Certificate Factory documentation