Fixed #21902 -- Documented search order for list_display.

      the HTML output, in the form of ``column-<field_name>`` on each ``<th>``

      element. This can be used to set column widths in a CSS file for example.

    * Django will try to interpret every element of ``list_display`` in this

      order:

      * A field of the model.

      * A callable.

      * A string representing a ``ModelAdmin`` attribute.

      * A string representing a model attribute.

      For example if you have ``first_name`` as a model field and

      as a ``ModelAdmin`` attribute, the model field will be used.

.. attribute:: ModelAdmin.list_display_links