Doc fix and clarification for n_slots in custom layout file

[crypdoughdoteth]

What I did

I updated the docs to fix the layout file example under Scoping and Declarations and clarify why n_slots is needed.

How I did it

I added a couple lines of text under the example and inserted the necessary n_slot information for the layout to work in the example.

How to verify it

Compile the example locally to ensure its correctness

Commit message

Doc fix and clarification for n_slots in custom layout file

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Description for the changelog

Documentation update to fix the layout file example under Scoping and Declarations and clarify why n_slots is needed.

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