Add info about handler return types to docstring

indexedDb/channels/handlers.go

indexedDb/channels/implementation.go

indexedDb/channels/init.go

indexedDb/dm/handlers.go

indexedDb/dm/implementation.go

indexedDb/dm/init.go

indexedDb/utils.go

indexedDbWorker/channels/init.go

indexedDbWorker/dm/init.go