feat(form-builder): improve labelling, give all fields descriptions
Per #646 (closed), all fields can now have a description beneath.
Per #647 (closed), simple changes to make the form-builder less confusing:
- Changed the labelling of "Field name" to "Name (internal field name)" and providing some extra instructions beneath;
- Changed the labelling of "Field short description" to "Short title (optional — used in concise listings)"
- Allow a description to be added below all field types (previously not supported for AuthorsInput or LinksInput)