Move `handlers` to the end of options example #docs #cosmetic

Now the order of the options in the example exactly matches the order in
which they're documented.
Add docs and error-checking for substituteTextarea
Comment statelessClipboard TODO

The current default behavior (new as of #269) is the one Desmos wants,
but the clipboard format being stateless was a sensible idea, I just
never properly implemented it, leave it as a TODO for now but at least
comment it.