Fixed #25687 -- Documented how to add database function support to third-party backends.

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Adding support in third-party database backends

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If you're using a database backend that uses a different SQL syntax for a

certain function, you can add support for it by monkey patching a new method

onto the function's class.

Let's say we're writing a backend for Microsoft's SQL Server which uses the SQL

``LEN`` instead of ``LENGTH`` for the :class:`~functions.Length` function.

We'll monkey patch a new method called ``as_sqlserver()`` onto the ``Length``

class::

    from django.db.models.functions import Length

    def sqlserver_length(self, compiler, connection):

        return self.as_sql(compiler, connection, function='LEN')

    Length.as_sqlserver = sqlserver_length

You can also customize the SQL using the ``template`` parameter of ``as_sql()``.

We use ``as_sqlserver()`` because ``django.db.connection.vendor`` returns

``sqlserver`` for the backend.

Third-party backends can register their functions in the top level

``__init__.py`` file of the backend package or in a top level ``expressions.py``

file (or package) that is imported from the top level ``__init__.py``.

For user projects wishing to patch the backend that they're using, this code

should live in an :meth:`AppConfig.ready()<django.apps.AppConfig.ready>` method.