fix 4.06 support: add floatarray predef type
fix 4.06 support: add floatarray predef type

Fixes ocaml-doc/doc-ock#69 and ocaml-doc/odoc#103

Minor changes included by @aantron to port the commit to the merged
repository:

- Commented out old comment representation. This will be replaced by a
  comment in the new representation, with the same content, in a
  follow-on commit.
- Some unused values have been prefixed with underscores, as the build
  in the merged repo has more severe warning settings for development.

ocaml-doc/odoc,ocaml-doc/odoc