Tweak param docs for Form.set_validator()

tailbone/forms/core.py

@@ -742,9 +742,8 @@ class Form(object):
            case the validator pertains to the form at large instead of
            one of the fields.
 
-           TODO: what should the validator look like?
-
-        :param validator: Callable validator for the node.
+        :param validator: Callable which accepts ``(node, value)``
+           args.
         """
         self.validators[key] = validator