Add support for the guidance form for labels. This is used to provide additional guidance about a question that isn't always needed and so is collapsed by default.

User docs: https://docs.getodk.org/form-language/, https://docs.getodk.org/collect-settings/#show-guidance-for-questions
Spec: bottom of https://getodk.github.io/xforms-spec/#languages

XLSForm

Collect

Enketo