Add documentation for -ppx

One thing that I'm not sure of is whether to include documentation for Ast_helper and most of its siblings in the pdf (also text &c) manual. Ast_helper alone takes twenty pages. This is not a problem for HTML manual.

I frequently look up signatures in the HTML manual, so it's useful even if it's not documented. However I'm not sure if that is the case with linear manuals.
I'm not in position to make a decision on the maintenance guarantees on these interfaces -- and we should discuss it elsewhere. All I know is that, for now, there exists none.

We cannot have a documentation that would suggest to users that the interface is more stable than it actually is. It would be an unfair burden both to unsuspecting users and to the maintainers. Pick whichever wording you think it best, but this documentation must make it absolutely clear that *these interfaces may change at the next OCaml relase*.