Convert comments into docstrings

[mithro]

You have a lot of comments like this;
https://github.com/duck2/uxsdcxx/blob/65c17a2b626e4d2429256db89b5496e2788ffa99/uxsdcxx.py#L456-L458

They should instead be written like this;

# May cause a bug: sets aren't guaranteed to be ordered.
def _gen_state_tables(t: XsdComplexType) -> str:
    """Generate state transition tables, indexed by token enums."""

Generally all functions should have these docstrings.

[duck2]

I have added some docstrings:
https://github.com/duck2/uxsdcxx/blob/25d8b1c799c87e48e0e268d73ee418b62a7e2bce/uxsdcxx.py#L474-L488