DOC: clarify matrix.ravel docstring and add tests

I had a difficult time tracking down exactly which code was being called by `m.ravel()` but I think this is right.
Actually I think you're right.  I'll change it.  The difference is that `core.multiarray.ndarray.ravel(self)` used to pass the tests without passing though the `order` argument, whereas `N.ndarray.ravel(self)` would not pass the tests.  Now that I've corrected the args to pass though the `order` arg, both pass the tests.  So I'll change it to use the `N.ndarray` form.