Tidy docs for Device API

[sftim]

Changes

Tidying for https://github.com/firecracker-microvm/firecracker/blob/main/docs/device-api.md

Reason

Minor cleanups for readability and context.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check CONTRIBUTING.md.

PR Checklist

• [x] If a specific issue led to this PR, this PR closes the issue.
• [x] The description of changes is clear and encompassing.
• [x] Any required documentation changes (code and docs) are included in this PR.
• [ ] API changes follow the Runbook for Firecracker API changes. not applicable
• [ ] User-facing changes are mentioned in CHANGELOG.md. not applicable
• [ ] All added/changed functionality is tested. not applicable
• [ ] New TODOs link to an issue. not applicable
• [x] Commits meet contribution quality standards.

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• [x] This functionality cannot be added in rust-vmm.

[sftim]

I haven't found time yet to redo this.

[roypat]

Hi @sftim, are you still interested in working on this?