document internal callback

src/allmydata/test/cli/test_cli.py

@@ -911,7 +911,13 @@ class Mkdir(GridTestMixin, CLITestMixin, unittest.TestCase):
             self.failUnlessReallyEqual(err, "")
             self.failUnlessIn(st, out)
             return out
+
         def _mkdir(ign, mutable_type, uri_prefix, dirname):
+            """
+            :param str mutable_type: 'sdmf' or 'mdmf' (or uppercase versions)
+            :param str uri_prefix: kind of URI
+            :param str dirname: the directory alias
+            """
             d2 = self.do_cli("mkdir", "--format={}".format(mutable_type), dirname)
             d2.addCallback(_check, uri_prefix)
             def _stash_filecap(cap):