Clarified the intermediary models admin documentation and a minor clean-up.

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By default, admin widgets for many-to-many relations will be displayed inline

on whichever model contains the actual reference to the `ManyToManyField`.  

on whichever model contains the actual reference to the ``ManyToManyField``.

However, when you specify an intermediary model using the ``through``

argument to a ``ManyToManyField``, the admin will not display a widget by 

default. This is because each instance of that intermediary model requires

        invite_reason = models.CharField(max_length=64)

The first step in displaying this intermediate model in the admin is to

define an inline model for the Membership table::

define an inline class for the ``Membership`` model::

    class MembershipInline(admin.TabularInline):

        model = Membership

        extra = 1

This simple example uses the defaults inline form for the Membership model,

and shows 1 extra line. This could be customized using any of the options

available to inline models.

This simple example uses the default ``InlineModelAdmin`` values for the

``Membership`` model, and limits the extra add forms to one. This could be

customized using any of the options available to ``InlineModelAdmin`` classes.

Now create admin views for the ``Person`` and ``Group`` models::

    admin.site.register(Person, PersonAdmin)

    admin.site.register(Group, GroupAdmin)

Now your admin site is set up to edit ``Membership`` objects inline from either

the ``Person`` or the ``Group`` detail pages.

Now your admin site is set up to edit ``Membership`` objects inline from

either the ``Person`` or the ``Group`` detail pages.

``AdminSite`` objects

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