Access entity ID in form using name property

https://forum.getodk.org/t/how-to-get-entity-uuid-id-in-xls-form/47122/3?u=ln

Start with the linked ODK forum discussion to determine the intended guidance and where it belongs in the documentation. Identify the relevant documentation entry point, then document how an entity ID can be accessed in an XLSForm using the name property; done means the guidance answers the linked question clearly.

Written by the indexing model from the issue text.