[1.10.x] Clarified URL regex check message with respect to include().

Backport of 7ed8b98f42 from master

django/core/checks/urls.py

@@ -97,8 +97,11 @@ def check_pattern_startswith_slash(pattern):
     regex_pattern = pattern.regex.pattern
     if regex_pattern.startswith('/') or regex_pattern.startswith('^/'):
         warning = Warning(
-            "Your URL pattern {} has a regex beginning with a '/'. "
-            "Remove this slash as it is unnecessary.".format(describe_pattern(pattern)),
+            "Your URL pattern {} has a regex beginning with a '/'. Remove this "
+            "slash as it is unnecessary. If this pattern is targeted in an "
+            "include(), ensure the include() pattern has a trailing '/'.".format(
+                describe_pattern(pattern)
+            ),
             id="urls.W002",
         )
         return [warning]

docs/ref/checks.txt

@@ -644,6 +644,8 @@ The following checks are performed on your URL configuration:
   including URLs.
 * **urls.W002**: Your URL pattern ``<pattern>`` has a ``regex``
   beginning with a ``/``. Remove this slash as it is unnecessary.
+  If this pattern is targeted in an :func:`~django.conf.urls.include`, ensure
+  the :func:`~django.conf.urls.include` pattern has a trailing ``/``.
 * **urls.W003**: Your URL pattern ``<pattern>`` has a ``name``
   including a ``:``. Remove the colon, to avoid ambiguous namespace
   references.

tests/check_framework/test_urls.py

@@ -46,7 +46,13 @@ class CheckUrlsTest(SimpleTestCase):
         self.assertEqual(len(result), 1)
         warning = result[0]
         self.assertEqual(warning.id, 'urls.W002')
-        expected_msg = "Your URL pattern '/starting-with-slash/$' has a regex beginning with a '/'"
+        expected_msg = (
+            "Your URL pattern '/starting-with-slash/$' has a regex beginning "
+            "with a '/'. Remove this slash as it is unnecessary. If this "
+            "pattern is targeted in an include(), ensure the include() pattern "
+            "has a trailing '/'."
+        )
+
         self.assertIn(expected_msg, warning.msg)
 
     @override_settings(ROOT_URLCONF='check_framework.urls.name_with_colon')