Fixed #36391 -- Doc'd RawSQL usage in docs/topics/db/sql.txt.

Author: aadeina

docs/topics/db/sql.txt

@@ -4,9 +4,11 @@ Performing raw SQL queries
 
 .. currentmodule:: django.db.models
 
-Django gives you two ways of performing raw SQL queries: you can use
-:meth:`Manager.raw` to `perform raw queries and return model instances`__, or
-you can avoid the model layer entirely and `execute custom SQL directly`__.
+Django gives you three ways of performing raw SQL queries: you can embed raw
+SQL fragments into ORM queries using
+:class:`~django.db.models.expressions.RawSQL` (see :ref:`raw-sql-fragments`),
+use :meth:`Manager.raw` to `perform raw queries and return model instances`__,
+or avoid the model layer entirely and `execute custom SQL directly`__.
 
 __ `performing raw queries`_
 __ `executing custom SQL directly`_
@@ -34,6 +36,19 @@ __ `executing custom SQL directly`_
     Please read more about :ref:`SQL injection protection
     <sql-injection-protection>`.
 
+.. _raw-sql-fragments:
+
+Raw SQL fragments
+=================
+
+In some cases, you may need to embed raw SQL fragments directly into ORM
+queries — for example, in :meth:`~django.db.models.query.QuerySet.annotate` or
+:meth:`~django.db.models.query.QuerySet.filter` calls. Use :ref:`Func()
+expressions <func-expressions>` for calling database functions across
+backends, or
+:class:`~django.db.models.expressions.RawSQL` for arbitrary parameterized SQL
+fragments.
+
 .. _executing-raw-queries:
 
 Performing raw queries
@@ -195,28 +210,6 @@ must always be included in a raw query. A
 :class:`~django.core.exceptions.FieldDoesNotExist` exception will be raised if
 you forget to include the primary key.
 
-Adding annotations
-------------------
-
-You can also execute queries containing fields that aren't defined on the
-model. For example, we could use `PostgreSQL's age() function`__ to get a list
-of people with their ages calculated by the database:
-
-.. code-block:: pycon
-
-    >>> people = Person.objects.raw("SELECT *, age(birth_date) AS age FROM myapp_person")
-    >>> for p in people:
-    ...     print("%s is %s." % (p.first_name, p.age))
-    ...
-    John is 37.
-    Jane is 42.
-    ...
-
-You can often avoid using raw SQL to compute annotations by instead using a
-:ref:`Func() expression <func-expressions>`.
-
-__ https://www.postgresql.org/docs/current/functions-datetime.html
-
 Passing parameters into ``raw()``
 ---------------------------------