[2.1.x] Refs #20910 -- Replaced snippet directive with code-block.

Backport of c49ea6f591 from master

docs/internals/contributing/writing-code/coding-style.txt

@@ -123,8 +123,8 @@ Imports
 
   For example (comments are for explanatory purposes only):
 
-  .. snippet::
-      :filename: django/contrib/admin/example.py
+  .. code-block:: python
+      :caption: django/contrib/admin/example.py
 
       # future
       from __future__ import unicode_literals

docs/internals/contributing/writing-code/unit-tests.txt

@@ -446,8 +446,8 @@ Since this pattern involves a lot of boilerplate, Django provides the
     :func:`~django.test.utils.isolate_apps` instances are correctly
     installed, you should pass the set of targeted ``app_label`` as arguments:
 
-    .. snippet::
-        :filename: tests/app_label/tests.py
+    .. code-block:: python
+        :caption: tests/app_label/tests.py
 
         from django.db import models
         from django.test import SimpleTestCase