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.
Sorry for being unclear: `-ppx` is not experimental, nor is the extension/attributes syntax. But the in-compiler interfaces (the `.mli` files) that authors rely on *are* experimental (even the `Parsetree` definitions, as far as I'm aware). The new opportunities offered by `compiler-libs` explicitly came with a "no guarantees so far" tag that hasn't been lifted yet.