[Elastic Agent] Better document the diagnostic upload requirements for fleet<->agent communication

Recently we were investigating collecting agent diagnostics from fleet not working. The agents were failing to upload the diagnostics due to limitations set on the components between the agent and fleet-server.

The diagnostics upload feature:

- uploads files up to 100MB,
- uploaded on 4BM chuncks,
- to the `/api/fleet/uploads/{id}/{chunkNum}` endpoint.

Therefore we should document this as requirements for the infrastructure between agents<->fleet-server.

The problem we investigated was due to the fleet-server ingress be configured to accept a max body size of 1MB. Similar issues might happen on on-prem deployments.