DOC-1281: edits for Puppet Server external_ca_config

Edited the external_ca_config doc for Puppet Server.
I'm not sure if the "whether it's an external CA or a Puppet CA" makes sense here.  The client certificate itself won't likely be that of a CA.  Instead, it would have been issued from a CA.  I'm not sure if this clause adds much value here anyway, so maybe it would be best to just delete it.