docs(delegation): add Go doc for Tag

delegation/schema.go

@@ -14,8 +14,14 @@ import (
 	"github.com/ucan-wg/go-ucan/pkg/meta"
 )
 
+// [Tag] is the string used as a key within the SigPayload that identifies
+// that the TokenPayload is a delegation.
+//
+// [Tag]: https://github.com/ucan-wg/delegation/tree/v1_ipld#type-tag
 const Tag = "ucan/dlg@1.0.0-rc.1"
 
+// TODO: update the above Tag URL once the delegation specification is merged.
+
 //go:embed delegation.ipldsch
 var schemaBytes []byte