[dune] [merlin] Fix some usability issues.

As suggested on Gitter by @maximedenes we improve documentation and
update Kernel's `.merlin` so it actually reports on the stricter set
of warnings.

We also set the language version to the proper one so users will get a
better error message [the fact that we can use `(env_var ...)` with
the wrong Dune version is a Dune bug indeed].

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