Clarify use of resolvers in code

What does this MR do?

I realised the documentation wasn't 100% clear that you could re-use resolver classes (that is, declare that the same resolver class could be used to resolve two different fields). What we want to avoid is instantiation and re-use of the same resolver object, which we need to leave up to the framework to manage.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

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