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

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