Clarify the family tables

[brendagutman]

1. Specify whether the table contains inverse mappings.

• Example 1

Subject1 | CHILD | Subject2
Subject2 | MTH | Subject1

2. Point to a source of truth for enumerations

• Suggestion: Point to the source that feeds MapDragon, or to a demo-locked down version of MapDragon.

[RobertJCarroll]

1: We can add some conventions in here.

2: This is caught up in the wider enumeration conversation. It's one place we'd like bindings a la FHIR, but that's not expressly supported unfortunately.