doc(net): remove instruction for nameserver setup via boot_args

[kalyazin]

Changes

Remove instruction for nameserver setup via boot_args.
Related issue: #5172 .

Reason

It does not work with our current rootfs.

License Acceptance

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PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
  - [ ] I have mentioned all user-facing changes in CHANGELOG.md.
  - [ ] If a specific issue led to this PR, this PR closes the issue.
  - [ ] When making API changes, I have followed the
  Runbook for Firecracker API changes.
  - [ ] I have tested all new and changed functionalities in unit tests and/or
  integration tests.
  - [ ] I have linked an issue to every new TODO.

---

• This functionality cannot be added in rust-vmm.