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