chore: more info about enum types in best practices

Author: david-roper

docs/en/5-reference/5.0-form-instrument.md

@@ -68,6 +68,9 @@ Always consider using \`group\`ing to structure sections
 8. Use [the playground](https://playground.opendatacapture.org/) to create instruments
    - It is updated regularly to use the latest runtime
    - It is setup with formatting, linting, and type checking by default
+9. Proper use of Zod's enum type
+   - It is best to use Zod's enum type when the answer to a form question is a set of constant values.
+   - Example: A question which asks the user to choose from a selection of options should use z.enum([option1, option2, ...]) within its zod validation schema.
 
 ### Example