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

docs/howto/custom-model-fields.txt

@@ -458,11 +458,14 @@ For example::
             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,