[REQUEST]: Document auto-unenrollment of Fleet Managed Elastic Agents

### Description

Document:
- Fleet-managed Elastic Agents will un-enroll after 7 consecutive "unauthenticated" errors received from Fleet Server (typically ApiKey invalid or very edge situations like proxies returning a 401 errors).
- The Elastic Agent will shut down after the un-enrollment.

Bonus:
- list reasons why an Elastic Agent might un-enroll (this + unenrollment timeout due to failed check-ins
- explain what happens during a check-in and how it can fail

Backport to any Fleet Agent version

### Resources

https://github.com/elastic/elastic-agent/blob/90167b9e65c7a9e0dbe4e9c92c43c767efdfeb0d/internal/pkg/agent/application/gateway/fleet/fleet_gateway.go#L302

https://github.com/elastic/elastic-agent/blob/90167b9e65c7a9e0dbe4e9c92c43c767efdfeb0d/internal/pkg/agent/application/gateway/fleet/fleet_gateway.go#L347

### Collaboration

Other (please describe below in the Point of contact section).

### Point of contact.

**Main contact:** Probably @joshdover ?