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

Thanks Kristof Claes for the initial patch.

docs/ref/models/expressions.txt

@@ -640,3 +640,34 @@ Let's see how it works::
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+
+Adding support in third-party database backends
+-----------------------------------------------
+
+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.