add a doc note

Author: aulemahal

docs/user/formatting.rst

@@ -98,7 +98,8 @@ formats:
    '2.3e-06 kilogram ** -1 * meter ** 3 * second ** -2'
 
 where ``unit`` is a :py:class:`dict` subclass containing the unit names and
-their exponents.
+their exponents, ``registry`` is the current instance of :py:class:``UnitRegistry`` and
+``options`` is not yet implemented.
 
 You can choose to replace the complete formatter. Briefly, the formatter if an object with the
 following methods: `format_magnitude`, `format_unit`, `format_quantity`, `format_uncertainty`,