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 don't have many problems hiding `Lexer` (it *is* ugly as hell) and `Printast` (a better `Printast` is one made with `[@@deriving show]` :p) for now. 

`Pprintast` is very useful for printing error messages, I have used it several times in my ppx extensions. `Parse` is useful for things like interactive toplevels and such. I think it should be present for completeness.