Merge branch 'bc/doc-compat-object-format-not-working' into maint-2.51

The compatObjectFormat extension is used to hide an incomplete
feature that is not yet usable for any purpose other than
developing the feature further.  Document it as such to discourage
its use by mere mortals.

* bc/doc-compat-object-format-not-working:
  docs: note that extensions.compatobjectformat is incomplete

Author: gitster

Documentation/config/extensions.adoc

@@ -14,6 +14,10 @@ compatObjectFormat::
 	compatObjectFormat.  As well as being able to use oids encoded in
 	compatObjectFormat in addition to oids encoded with objectFormat to
 	locally specify objects.
++
+Note that the functionality enabled by this extension is incomplete and subject
+to change.  It currently exists only to allow development and testing of
+the underlying feature and is not designed to be enabled by end users.
 
 noop::
 	This extension does not change git's behavior at all. It is useful only