--- a/mercurial/configitems.py	Fri Nov 25 15:14:40 2022 +0100
+++ b/mercurial/configitems.py	Mon Oct 17 16:26:16 2022 +0200
@@ -2075,6 +2075,7 @@
     b'revlog.reuse-external-delta',
     default=True,
 )
+# This option is True unless `format.generaldelta` is set.
 coreconfigitem(
     b'storage',
     b'revlog.reuse-external-delta-parent',

--- a/mercurial/scmutil.py	Fri Nov 25 15:14:40 2022 +0100
+++ b/mercurial/scmutil.py	Mon Oct 17 16:26:16 2022 +0200
@@ -1858,7 +1858,12 @@
 
 
 def gddeltaconfig(ui):
-    """helper function to know if incoming delta should be optimised"""
+    """helper function to know if incoming deltas should be optimized
+
+    The `format.generaldelta` config is an old form of the config that also
+    implies that incoming delta-bases should be never be trusted. This function
+    exists for this purpose.
+    """
     # experimental config: format.generaldelta
     return ui.configbool(b'format', b'generaldelta')