PagerDuty does not maintain the ElastAlert documentation, nor can we assume responsibility for how they document their integrations with other platforms.
Regarding your second inquiry, to set the description on a ServiceNow incident that is triggered from PagerDuty, I would suggest constructing an inbound field rule. You can find the inbound field rules in ServiceNow under PagerDuty → Configuration → Inbound Field Rules. You will want to set PagerDuty webhook type to Trigger and ServiceNow incident field to Description. With the Type setting, you can choose between setting a default description or basing the description off of a field in the PagerDuty webhook payload.
I hope this helps!