what is masked_reference useful for outside ReadRel?

[benbellick]

masked_reference in FieldReference is used in ReadRel.projection to select subsets of fields. However, it's unclear what FieldReference with masked_reference means in every context it could be used in.

1. What is masked_reference useful for outside of ReadRel.projection?
2. Can we document concrete use cases, or should we restrict/remove it from FieldReference?
3. Should lambda parameters forbid masked_reference (what contexts would it make sense to use masked_reference for lambdas)?

If it doesn't unlock a concrete query form and isn't being used by anyone, it may be worth considering dropping it for the sake of simplifying the spec pre-1.0.

Originally raised in PR #955