docs: Document the ability to use prefix in dynamic sampler FieldList

[irvingpop]

Short description of the changes

• This PR documents the feature from feat: allow root. in field list for dynamic sampler #1275, showing our users how to specify the root. prefix in FieldLists everywhere
• It also sprinkles the use of this feature around the rules_complete.yaml example

[kentquirk]

Thanks.

For future reference, we generally don't check in the autogenerated files for intermediate PRs; they get built as one of the steps in our release protocol. No harm done, but it minimizes the number of things to review.