Change documentation comments’ style

Change file/class/member/method/function documentation comments to use the
special delimiters /** */ instead of ## for the first line. This hopefully
makes comments more readable, as it seems to be the case with the
similarly-styled doc comments in Python, and it also makes doc comments
more easily recognizable by a parser.

Also add TODOs to document signatures (arguments and return values), just
like I do for every other project I work on.