Changes: attribution information for 4.01

I had collected attribution information for the announce only, but
Damien agreed to put them in the Changes file as well.

This change also fixes minor problems with the Changes content:
- type-based disambiguation of constructors and labels was not mentioned
- the open! syntax was not mentioned
- PR#5552 was mislabeled
- PR#5758 (matching on floats) was announced despite being also
  present in the 4.00.1 changes
- I moved PR#5986 (Marshal.Compat_32) from 'bug' to 'feature'
- PR#5243 was absent (ocamlbuild API documentation)

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14089 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02