Add IncidentDetails.Title field and mark IncidentDetails.Description as deprecated

[theckman]

Refer to this line of code:

go-pagerduty/webhook.go

Line 31 in 1df5787

Description string `json:"description"`

The PagerDuty V2 Webhooks documentation indicates that the description field is deprecated in-favor of the title field. We need to add a Title field to the struct, and add a comment above the Description field to indicate it's deprecated:

[theckman]

Looks like I didn't scroll up. Title is already there, just needs a comment.