missed one docstring

Author: jernejfrank

hamilton/function_modifiers/macros.py

@@ -1113,6 +1113,15 @@ def __init__(
         _chain: bool = False,
         **mutating_function_kwargs: Union[SingleDependency, Any],
     ):
+        """Instantiates a `@mutate` decorator.
+
+        We assume the first argument of the decorated function to be the output of the function we are targeting.
+
+        :param target_functions: functions we wish to mutate the output of
+        :param collapse: Whether to collapse this into a single node. This is not currently supported.
+        :param _chain: Whether to chain the first parameter. This is the only mode that is supported. Furthermore, this is not externally exposed. @flow will make use of this.
+        :param **mutating_function_kwargs: other kwargs that the decorated function has
+        """
         self.collapse = collapse
         self.chain = _chain
         # keeping it here once it gets implemented maybe nice to have options