[1.8.x] Fixed #25213 -- Discouraged use of QuerySet.extra()

Thanks Anssi Kääriäinen for the draft text and Simon Charette
for review.

Backport of e8cd65f829 from master

docs/ref/models/querysets.txt

@@ -1136,6 +1136,34 @@ Sometimes, the Django query syntax by itself can't easily express a complex
 ``QuerySet`` modifier — a hook for injecting specific clauses into the SQL
 generated by a ``QuerySet``.
 
+.. admonition:: Use this method as a last resort
+
+    This is an old API that we aim to deprecate at some point in the future.
+    Use it only if you cannot express your query using other queryset methods.
+    If you do need to use it, please `file a ticket
+    <https://code.djangoproject.com/newticket>`_ using the `QuerySet.extra
+    keyword <https://code.djangoproject.com/query?status=assigned&status=new&keywords=~QuerySet.extra>`_
+    with your use case (please check the list of existing tickets first) so
+    that we can enhance the QuerySet API to allow removing ``extra()``. We are
+    no longer improving or fixing bugs for this method.
+
+    For example, this use of ``extra()``::
+
+        >>> qs.extra(
+        ...     select={'val': "select col from sometable where othercol = %s"},
+        ...     select_params=(someparam,),
+        ... )
+
+    is equivalent to::
+
+        >>> qs.annotate(val=RawSQL("select col from sometable where othercol = %s", (someparam,)))
+
+    The main benefit of using :class:`~django.db.models.expressions.RawSQL` is
+    that you can set ``output_field`` if needed. The main downside is that if
+    you refer to some table alias of the queryset in the raw SQL, then it is
+    possible that Django might change that alias (for example, when the
+    queryset is used as a subquery in yet another query).
+
 .. warning::
 
     You should be very careful whenever you use ``extra()``. Every time you use

docs/topics/db/optimization.txt

@@ -130,12 +130,12 @@ For instance:
 
 If these aren't enough to generate the SQL you need:
 
-Use ``QuerySet.extra()``
-------------------------
+Use ``RawSQL``
+--------------
 
-A less portable but more powerful method is
-:meth:`~django.db.models.query.QuerySet.extra()`, which allows some SQL to be
-explicitly added to the query. If that still isn't powerful enough:
+A less portable but more powerful method is the
+:class:`~django.db.models.expressions.RawSQL` expression, which allows some SQL
+to be explicitly added to the query. If that still isn't powerful enough:
 
 Use raw SQL
 -----------