Display deprecated attributes

[rgrinberg]

Items marked with [@@deprecated ..] and [@@@ocaml.deprecated ..] should be displayed as such in generated documentation.

[dbuenzli]

I always thought @deprecated tags were lifted into annotations by the compiler. That doesn't seem to be the case.

This makes the whole thing non-dry for devs.

Would it therefore be possible to treat the alert message as an ocamldoc comment for the structure item ?