Make technique of using required note as a constraint more prominent

https://docs.getodk.org/form-logic/#validating-and-restricting-responses

https://forum.getodk.org/t/submissions-missing-in-odk-central/44919/5?u=ln -- it's particularly useful for cases like that one where there isn't another place to put a constraint.

Read the “Validating and restricting responses” section at docs.getodk.org/form-logic/ and review the linked ODK Forum example. Identify where the required-note constraint technique is currently described, then make that technique more prominent for cases with no other place to put a constraint. Done means the documentation clearly highlights this use case.

Written by the indexing model from the issue text.