This PR adds support for documentation comments in the following positions:

1. on the parameters of a function type:
   type 'a with_optionnal = ?optionnal:unit (** optionnal! *) -> 'a
val f : ?foo:int (** default [0] *) -> bar:string (** doc *) -> unit (** doc *) -> unit
2. inside polymorphic variant types:
   type t = [ Foo (** foo! *) | Bar of int * string (** bar? *) ]
3. inside object types:
   type t = < foo: int -> int (** foo *) ; bar: int -> int; (** bar *) .. >

Currently if warning 50 is enabled it will be emitted for every comment of the examples above.
This is unfortunate because people have not only been asking support for this for a while (see various issues on mantis), but have also been using that style while waiting for tooling support, see:

• examples of polymorphic variants documentation: here and there
• examples of function parameters annotations: here and there,
• example of an object type definition with annotations on methods: here

One should note that the warning inside object types as support was added in ocamldoc with this commit (cf. mantis PR#6274).

The other two cases were not supported but remove an unnecessary limitation and conforms with the style people are already using.