Fixed #5726 -- Documented that inspectdb doesn't detect defaults.

``inspectdb`` works with PostgreSQL, MySQL and SQLite. Foreign-key detection

only works in PostgreSQL and with certain types of MySQL tables.

Django doesn't create database defaults when a

:attr:`~django.db.models.Field.default` is specified on a model field.

Similarly, database defaults aren't translated to model field defaults or

detected in any fashion by ``inspectdb``.

By default, ``inspectdb`` creates unmanaged models. That is, ``managed = False``

in the model's ``Meta`` class tells Django not to manage each table's creation,

modification, and deletion. If you do want to allow Django to manage the