refine ocamlbuild/.depend for `make -jN` to work again

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14576 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Added comments to parsetree.mli

(Patch by Yotam Barnoy)

Specifically, I documented some more obscure features that weren't
obvious from reading the existing comments.

Discussion of these comments can be found on the mailing list at
  https://sympa.inria.fr/sympa/arc/caml-list/2014-04/msg00022.html

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14577 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02