Formula Cookbook: Enrich the Service block methods section

[Jerry1144]

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Expand the explanation of why run is required for most fields in service do, and expand the description of name and require_root. Following #16908 and the doc change in #16914

[MikeMcQuaid]

Looks great, thanks @Jerry1144! One tiny nit.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @Jerry1144!