api: add missing docstring

qubes/api/__init__.py

@@ -51,6 +51,8 @@ def method(name, *, no_payload=False, endpoints=None, **classifiers):
     :param str name: qrexec rpc method name
     :param bool no_payload: if :py:obj:`True`, will barf on non-empty payload; \
         also will not pass payload at all to the method
+    :param iterable endpoints: if specified, method serve multiple API calls
+        generated by replacing `{endpoint}` with each value in this iterable
 
     The expected function method should have one argument (other than usual
     *self*), ``untrusted_payload``, which will contain the payload.