* lisp/org-element.el (org-element--cache-diagnostics-modifications):
New variable to enable/disable warnings when unregistered
modifications are detected in Org buffers.
* lisp/org-element.el (org-element--cache-sync): Only show
"Unregistered modification" warning when
`org-element--cache-diagnostics-modifications' is non-nil or during
Org tests. Add special handling for unregistered changes by
`replace-match' in Emacs <28.
Fixes https://list.orgmode.org/87pmr6lu1y.fsf@localhost/T/#t