ocaml-cbor: enable MDX on lib/value.mli, fix broken doc example
The Value example used 'Cbor.Map'/'Cbor.Text' (top-level constructors
that don't exist) and 'Cbor.int 30' (no such smart constructor). Open
Cbor at the top of the block, qualify the constructors as
Value.Map/Value.Text, and replace 'Cbor.int 30' with 'Value.Int
(Z.of_int 30)' which is the actual variant + zarith int. Add an
'assert false'-guarded match so the build catches drift in the Value.t
shape.