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.
Actually, I disagree completely. Generated **HTML** documentation would be incredibly useful; you don't see these things unless you go through the link and it is immeasurably better than browsing .mli file in every way. I strongly prefer as much .mli files going into HTML as possible.

I'm only worried about pdf/text manuals here. So, I think it is a good idea to only include the better-documented parts into the pdf/text manual--with your points 1-3, it would be everything except Ast_helper.