Add documentation of GraphQL query validation for FE developers

We should explain how we validate queries, what assumptions we currently make and how things would need to change if those assumptions change.

Specifically in regards to client queries, if we start using mixed queries. then we will need to stop skipping so aggressively.

The following discussion from !50655 (merged) should be addressed:

• @engwan started a discussion: (+1 comment)

  Looking at https://www.apollographql.com/docs/tutorial/local-state/#query-local-fields, I see that the client directive is per-field. But here, we're skipping validation for the whole query.

  I know it makes it more complicated to just strip the client-side fields so I guess this is fine for now. I think we also currently do not have queries that have mixed fields.