--- a/mercurial/help.py	Sun Nov 04 12:51:14 2018 +0900
+++ b/mercurial/help.py	Mon Nov 12 21:28:54 2018 +0900
@@ -311,6 +311,8 @@
      loaddoc('changegroups', subdir='internals')),
     (['config'], _('Config Registrar'),
      loaddoc('config', subdir='internals')),
+    (['extensions', 'extension'], _('Extension API'),
+     loaddoc('extensions', subdir='internals')),
     (['requirements'], _('Repository Requirements'),
      loaddoc('requirements', subdir='internals')),
     (['revlogs'], _('Revision Logs'),