Documented jQuery upgrade

Django admin Javascript makes use of the `jQuery`_ library.

To avoid conflicts with user-supplied scripts or libraries, Django's jQuery

(version 1.4.2) is namespaced as ``django.jQuery``. If you want to use jQuery

(version 1.9.1) is namespaced as ``django.jQuery``. If you want to use jQuery

in your own admin JavaScript without including a second copy, you can use the

``django.jQuery`` object on changelist and add/edit views.

.. versionchanged:: 1.6

    The embedded jQuery has been upgraded from 1.4.2 to 1.9.1.

The :class:`ModelAdmin` class requires jQuery by default, so there is no need

to add jQuery to your ``ModelAdmin``'s list of media resources unless you have

a specifc need. For example, if you require the jQuery library to be in the

* Added the :attr:`django.db.models.ForeignKey.db_constraint`

  option.

* The jQuery library embedded in the admin has been upgraded to version 1.9.1.

Backwards incompatible changes in 1.6

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