Doxygen: TODO cleanup, use @todo where needed.

Doxygen: TODO cleanup, use @todo where needed.

Turn TODOs which should be user-visible into @todo so that Doxygen
shows them in the function docs, and also on the special "Todo List" page.

Those TODOs that should not be in the Doxygen docs are moved out of the
/** */ comment blocks.

Also fix some comments/items, and remove some obsolete ones.