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.
Documenting more modules is a mildly controversial change. I personally think it's a good idea, but maybe we should be modest in the amount of modules we document. Here is my understanding of the space, in *decreasing* order of (sujective) importance:

1. `Ast_{helper,mapper}` should clearly go in
2. `Parsetree` would be a natural choice given the excellent documentation work Alain has done (why is `parsetree.mli` seemingly missing here?)
3. `Asttypes`, `Longident` and `Location` are currently necessary to be construct ASTs
4. `Parse` and `Pprintast` are not necessary for all uses of `-ppx`, but could be mildly useful for some hackish purposes
5. I think we don't need `Lexer` and `Printast`

I think it would be reasonable to have (1,2,3) in, and maybe leave (4) aside for the moment -- we can always add more later.