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'll ask around for opinions on this, but for now I'm fine with going with what you think is best.

I still don't see the point of exposing `Lexer`, which is ugly as hell.

There would be some (minor) work to do to improve `pprintast.mli` if/when it is exposed, putting the `printer` class at the end (most users don't need it) and briefly describing the purpose in a top comment. (Not saying you personally should do this.)