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.
Non-backward-compatible way, that's obvious (but has to be specified). Experimental? I'm not so sure, -ppx is shipped in a release and so far everyone agrees it's the way forward. I don't think it's fair to say that -ppx is merely experimental, anymore.