Official doc missing points for suspend/resume Machines

[AshitaGK]

Title: Update doc on suspending/resuming Machines

Issue: Couldn't find the following information in the official doc which is relevant w.r.t suspending a Machine. They are:

• Machines cannot have swap enabled. suspend doesn't support swap_size_mb in fly.toml

• Machines must not have a schedule.

• GPU Machines cannot be suspended.

[AshitaGK]

For context, here's the link : https://community.fly.io/t/new-feature-in-preview-suspend-resume-for-machines/20672/15