docs(spec): indicate in detail how Channel Item Object field resolves

[char0n]

Refs #612

This PR targets targets two things:

1. As we now have reusable Channel Item Objects stored in Components.channels, there is no reason why the description of the $ref field should explicitly only allow referencing external definition of the Channel Item Object (in different document) and not the internal one

2. Second change targets the unspecified rules of resolution for the $ref field. I've added explicit mention of JSON Reference specification which the resolution of the $ref field is based on.

[sonarqubecloud]

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[char0n]

Succeeded by #779