Document runtime representation of extensible variants and exceptions

[sim642]

The runtime memory representation of extensible variants is not documented in the "Interfacing C with OCaml" section of the official manual nor the "Memory Representation of Values" section of RWO.
It is described by @Octachron on StackOverflow: https://stackoverflow.com/a/54872172/854540. This PR adds this information to the official manual because that SO answer is hard to find when looking for this information.

[dra27]

I'm struggling to find it, but I thought that in general when the question of these representations comes up, the answer has been not to deconstruct them from C (I think it's a similar story for the representation of first class modules and so forth). If that's the case, might that be a better explanation of that here? i.e. show you to use the Obj module instead?