Improve help on fastify start

[rommelandrea]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

According to the code, i found a lack on help for command fastify start.
Going to read issue #200 and within the code it is possible to enable debugging (inspect) and change the port, but within the help this possibility is not mentioned.
It's possible to improve the help like the image?

I can create the pull request, but there are any other activities that I need to do for this type of change that does not change the functionality of the project?

Motivation

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Example

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