* lisp/org.el (org-todo): Make a tripple `C-u' prefix force
logging the todo state change with timestamp and a note.
Ignoting blocking a change now needs four `C-u' prefixes,
because this is probably hardly ever used.
* doc/org-manual.org (Tracking TODO state changes): Document
forcing of state change logging with `C-u C-u C-u C-c C-t'.