Added omitted documentation for ArrayField__len.

    >>> Post.objects.filter(tags__overlap=['thoughts', 'tutorial'])

    [<Post: First post>, <Post: Second post>, <Post: Third post>]

.. fieldlookup:: arrayfield.len

len

~~~

Returns the length of the array. The lookups available afterwards are those

available for :class:`~django.db.models.IntegerField`. For example::

    >>> Post.objects.create(name='First post', tags=['thoughts', 'django'])

    >>> Post.objects.create(name='Second post', tags=['thoughts'])

    >>> Post.objects.filter(tags__len=1)

    [<Post: Second post>]

.. fieldlookup:: arrayfield.index

Index transforms