There's a note in the help file that comes up with that
-h option that says you can go
here for full documentation of the conversion system, but it's not up yet.
Oh, and so far, I'm very much liking the new ebook-convert program; it lets me set the language and rating and ISBN and series name and other metadata that the old
html2oeb wouldn't let me specify. (I don't know if the old version of
ebook-convert would do that too, since this is the first time I'm using it, but I just wanted to mention that it's pretty cool so far.)