chore: add best practice about select variant string fields having enum validaitonSchema

Author: david-roper

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

@@ -67,6 +67,7 @@ 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. Whenever a form field of kind 'string', and of variant 'select' is used, it is much better to have its schema be of type enum containing the possible options instead of type string ( i.e. z.enum(['string1', 'string2', ...]) instead of z.string() ).
 
 ### Example