PostGraphile docs: ambiguous example in makeExtendSchemaPlugin docs
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### Summary

https://postgraphile.org/postgraphile/next/make-extend-schema-plugin#example-2 needs fixing, add some warnings around it for context

### Additional context

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One of our sponsors has asked if there's a potential N+1 problem within this example, which Benjie has described as "terrible". 

> `withPgClient` by default is designed for use in mutations. It runs the query once for each entry in the batch. At the root level (as here) there is no `N` (we're at root) so this is fine; but if you were to add that to a non-root type (e.g. `User` or `Post` or whatever) then yes absolutely it would destroy performance. Further `withPgClient` is flagged as a mutation (`hasSideEffects = true`) and that will impact the efficiency of your plans too. 
