Fixed #21478 -- Corrected docs for when Field.db_type() is called.

            else:

                return 'timestamp'

The :meth:`.db_type` method is only called by Django when the framework

The :meth:`.db_type` method is called by Django when the framework

constructs the ``CREATE TABLE`` statements for your application -- that is,

when you first create your tables. It's not called at any other time, so it can

afford to execute slightly complex code, such as the

``connection.settings_dict`` check in the above example.

when you first create your tables. It is also called when constructing a 

``WHERE`` clause that includes the model field -- that is, when you retrieve data

using QuerySet methods like ``get()``, ``filter()``, and ``exclude()`` and have

the model field as an argument. It's not called at any other time, so it can afford to

execute slightly complex code, such as the ``connection.settings_dict`` check in

the above example.

Some database column types accept parameters, such as ``CHAR(25)``, where the

parameter ``25`` represents the maximum column length. In cases like these,