Revert "No longer using "@todo" Doxygen command."

This reverts commit 140fb23d697793a3432c43f4aef5b4486eb39ada.

We actually do need to use @todo, to prevent Doxygen including the
text of the TODO in the generated documentation. However, the
actual todo list page is not generated by Doxygen, as this option is
still set to "NO" in Doxyfile.