Clarify Elastic Agent upgrade rollouts

[jillguyonnet]

Adding a couple of clarifications regarding upgrading Elastic Agents with a rollout period:

1. Agents in the rollout but not actually upgrading yet have status Updating in Fleet.
2. For auto-upgrades, the rollout period is automatically calculated based on the number of agents (less than 10 agents: no rollout; more than 10 agents: rollout duration of 10 min or nAgents * 0.03 seconds, whichever is higher).

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/upgrade-elastic-agent.md

[vishaangelova]

Left a comment for your consideration, but otherwise LGTM.