BlockInspector: Add showNoBlockSelectedMessage prop documentation.

[SainathPoojary]

related to #22891

What?

This PR adds missing documentation for the showNoBlockSelectedMessage prop in the BlockInspector component.

Why?

The showNoBlockSelectedMessage prop was introduced but lacked documentation. Adding this prop to the README ensures that developers understand its purpose and usage when working with the BlockInspect

Testing Instructions

1. Navigate to the BlockInspector component's README file.
2. Verify that the showNoBlockSelectedMessage prop is documented correctly.
3. Ensure that the prop’s description is clear, including its default value and type.

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Co-authored-by: SainathPoojary <[email protected]>
Co-authored-by: t-hamano <[email protected]>

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[SainathPoojary]

Hey @t-hamano,

The README was missing documentation for the showNoBlockSelectedMessage prop. I’ve added it in the PR. Could you please review the changes when you have a moment?

Thanks!

[t-hamano]

Nice catch!

[SainathPoojary]

Thank you for catching that, @t-hamano! I’ve updated the Markdown list to use the recommended 3 spaces for consistency. Let me know if there’s anything else that needs attention.

[Mamaduka]

I think we should just remove this prop. It hasn't been used anywhere in the the core for a while. Here's the PR: #68487.