admin api: add comments for InspectObject

doc/api/garage-admin-v2.json

@@ -1090,6 +1090,9 @@
               }
             }
           },
+          "404": {
+            "description": "Object not found"
+          },
           "500": {
             "description": "Internal server error"
           }

src/api/admin/openapi.rs

@@ -525,6 +525,7 @@ upload is in progress and not yet finished.
     params(InspectObjectRequest),
 	responses(
             (status = 200, description = "Returns exhaustive information about the object", body = InspectObjectResponse),
+            (status = 404, description = "Object not found"),
             (status = 500, description = "Internal server error")
         ),
 )]