Fixed #19712 - Clarified admindocs regarding which attributes appear.

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The **models** section of the ``admindocs`` page describes each model in the

system along with all the fields and methods available on it. Relationships to

other models appear as hyperlinks. Descriptions are pulled from ``help_text``

attributes on fields or from docstrings on model methods.

system along with all the fields and methods (without any arguments) available

on it. While model properties don't have any arguments, they are not listed.

Relationships to other models appear as hyperlinks. Descriptions are pulled

from ``help_text`` attributes on fields or from docstrings on model methods.

A model with useful documentation might look like this::