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 feel like you should give a sort of "reading order" (`Ast_mapper` first, for ppxs basic blocs, then `Parsetree` to see what the AST look like, then `Ast_helper`, to actually build pieces of asts).