diff --git a/mercurial/help.py b/mercurial/help.py
--- a/mercurial/help.py
+++ b/mercurial/help.py
@@ -30,6 +30,7 @@
     templatefilters,
     templatefuncs,
     templatekw,
+    ui,
     util,
 )
 from .hgweb import (
@@ -436,6 +437,11 @@
 addtopicsymbols('templates', '.. functionsmarker', templatefuncs.funcs)
 addtopicsymbols('hgweb', '.. webcommandsmarker', webcommands.commands,
                 dedent=True)
+def docplaceholder():
+    pass
+docplaceholder.__doc__ = ui.tweakrc
+addtopicsymbols('config', '.. tweakdefaultsmarker', {'': docplaceholder},
+                dedent=True)
 
 def help_(ui, commands, name, unknowncmd=False, full=True, subtopic=None,
           **opts):
diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -2370,6 +2370,10 @@
     effect if ``HGPLAIN`` is set or ``HGPLAINEXCEPT`` is set and does
     not include ``tweakdefaults``. (default: False)
 
+    It currently means::
+
+      .. tweakdefaultsmarker
+
 ``username``
     The committer of a changeset created when running "commit".
     Typically a person's name and email address, e.g. ``Fred Widget