Document --two-level

[nad]

The flag --two-level is not mentioned in the documentation.

[mikeshulman]

As of #6258 its existence is at least mentioned. But apparently it is still not described anywhere.

[andreasabel]

Maybe it was too experimental, so the contributors didn't give it citizen rights. But since it is still here, would be nice if someone volunteered a documentation.

[mikeshulman]

It's probably a bit more stable than it was originally, now that a few of us have started actually using it and some bugs have been reported and fixed. And other experimental options do appear in the reference manual.

However, it would still be nice if something could be done about #5761, particularly the point about the empty type here that doesn't actually involve cumulativity (should I report that as a separate bug?).