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.
But -ppx is literally useless without the in-compiler interfaces. You need at the very least Parsetree &co, and there's also a good reason I removed "helper" from ast_mapper.mli.

I think there's middle ground between "experimental" and "stable" that we need. It is clear that the compiler cannot provide a stable interface here without stopping evolving. However it is also clear that this interface will not, say, be abruptly removed, which would be only natural for an experimental one.

I think there should be a notice along the lines of "this interface follows the evolution of OCaml language and implementation, and thus does not provide any backwards compatibility guarantees at all"