[wg-camlp4] Structure/signature attributes suggestion

[Leo White]

[Sorry for repeat Alain: I accidently dropped wg-camlp4 from the reply]

> What about using regular item attributes for section headers?  This might be not so nice conceptually, but would there
> be actual problems with this approach?

For my use case it is probably ok, although I would prefer a cleaner
system. I imagine at some point someone will get a surprising section
header in the wrong place due to an incorrectly written ppx processor,
but perhaps that is fine.