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 wonder to which extent it wouldn't be better to just show the `ocamlfind` way directly. Or, to respect the manual tradition to be self-contained and teach us the hard way, at least follow it with the ocamlfind commands people will actually use (and adapt in their build systems).

(This also has technical advantages: the widely-documented habit to use `+camlp4` has become problematic now that camlp4 is split outside the distribution, so de-emphasizing the `+foo` feature seems reasonable to avoid such issues in the future.)