ocp-index : make the output a bit nicer.

This solve one of the point in my long list.
The output was : 
```ocaml
val Eliom_parameter.prod ('a, [ `WithoutSuffix ], 'b)
                         Eliom_parameter.params_type ->
                         ('c, [< `Endsuffix | `WithoutSuffix ] as 'e, 'd)
                         Eliom_parameter.params_type ->
                         ('a * 'c, 'e, 'b * 'd) Eliom_parameter.params_type
```
And is now :
```ocaml
val Eliom_parameter.prod
    ('a, [ `WithoutSuffix ], 'b) params_type ->
    ('c, [< `Endsuffix | `WithoutSuffix ] as 'e, 'd) params_type ->
    ('a * 'c, 'e, 'b * 'd) params_type
```

I tried to solve other issues in the same swing, but I'm not very comfortable with format yet. It should work correctly with documentation and small signatures.

In particular, I don't really know how Format know the size of the terminal, but I think he get it wrong.