docs: Clarify first boot registration service

[m-horky]

The original text roughly contained the steps to set up a service to register with on the next (first) boot, but contained some minor issues. Aside from typos/phrasing:

• It talked of 'startup' and 'next boot', but effectively this would only get executed on first boot.
• Command touch .run_next_boot was contained twice.

[henrywang]

Just make a rebase for test install failure.