Clarified custom lookups output_field documentation

this here as it didn't change, but supposing we were applying ``AbsoluteValue``

to some field which represents a more complex type (for example a point

relative to an origin, or a complex number) then we may have wanted to specify

``output_field = FloatField``, which will ensure that further lookups like

``abs__lte`` behave as they would for a ``FloatField``.

that the transform returns a ``FloatField`` type for further lookups. This can

be done by adding an ``output_field`` attribute to the transform::

    from django.db.models import FloatField, Transform

    class AbsoluteValue(Transform):

        lookup_name = 'abs'

        def as_sql(self, qn, connection):

            lhs, params = qn.compile(self.lhs)

            return "ABS(%s)" % lhs, params

        @property

        def output_field(self):

            return FloatField()

This ensures that further lookups like ``abs__lte`` behave as they would for

a ``FloatField``.

Writing an efficient abs__lt lookup

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~