Roles have a two-level structure: the role name is a parametric role
group, which in turns contains the parametric role. In the case of there
being many roles of the same short name, they are within the one group.
Declarator docs correctly attach to the parametric role. However, that
means .WHY on the group doesn't actually result in those docs. You have
to extract the candidate from the group.

In many cases, however, we people ain't using parametric roles, and we
make many meta-operations delegate to the single or unsignatured role
candidate, if any. We do not currently do this for .WHY. This commit
makes us do so, BUT it turns out there are 3 current spectests that will
fail because they explicitly expect that not to happen. Thus, we need to
make a decision over whether we want this change.

Relates to #3521.

- Remove `Perl6::Metamodel::Documenting` from consumed roles
- Fully implement `WHY`, `set_why`
- Make `set_why` throw `X::Role::Group::Documenting` as a safety measure

I deliberately don't delegate `set_why` in order to avoid accidental
overriding of parametric role documentation. It doesn't and must not
happen in Rakudo implementation anyway.

Because signature is a local scope from documentation point of view.

Fixes documenting of parametric role:

    role R[::T] {
        #= this now belongs to the role, not to the parameter T
    }

And makes one spec TODO passing.