Update docs to provide better clarity of supported features

[xmarcalx]

Signed-off-by: Marco Cali [email protected]

Reason for This PR

Modify the documentation to include explicitly the max number of vCPU supported and other minor grammatical changes.

Fixes #3059

Description of Changes

Updated the "Features" section of the design.md file to state the max vCPU number.

• [ ] This functionality can be added in rust-vmm.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

PR Checklist

• [X] All commits in this PR are signed (git commit -s).
• [X] The issue which led to this PR has a clear conclusion.
• [X] This PR follows the solution outlined in the related issue.
• [X] The description of changes is clear and encompassing.
• [X] Any required documentation changes (code and docs) are included in this PR.
• [ ] Any newly added unsafe code is properly documented.
• [ ] Any API changes follow the Runbook for Firecracker API changes.
• [ ] Any user-facing changes are mentioned in CHANGELOG.md.
• [ ] All added/changed functionality is tested.