The AGOL Dataset attributes should be editable. It looks great but contains little to no information. Ideally it would pull from the oracle data dictionary(field comments) (or other database equivalents). Better still you should be able to edit the field comments in Arcatalog/Desktop and they should port over to AGOL so you don't have to type them in more than once. Field level metadata has always been lacking in ArcGIS and even when it is added as comments on the database level it doesn't pull over in to the ArcGIS world.