adding verbose rustdoc to diff between build-spec and chain-spec-builder (#7698)

Follow up from #7619 
Adding a more verbose rustdoc for guidance and difference between
chain-spec-builder and build-spec commands under polkadot-omni-node

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Co-authored-by: Iulian Barbu <14218860+iulianbarbu@users.noreply.github.com>
Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>