Minimize markup in variand and field descriptions.

[aantron]

ocsigen.org's styles and parser currently assume that variand and field descriptions are simple preformatted comment strings. Meanwhile, wikidoc, like ocamldoc, expects the descriptions to be table cells with full documentation styling and block content.

This commit changes wikidoc to output the descriptions as text close to what is written in the source code, inserting much less markup.

I inspected the new output, and it looks correct up to my understanding of Wiki Creole and Ocsigen's extensions. I haven't tested final display on ocisgen.org.

I think the "right way" to fix this is to change the style and layout on ocsigen.org to allow block content in variand info, like ocamldoc does. However, that seems like an involved project that I'm not ready to do right now. Perhaps that can be done at a later date.

In the meantime, this helps to fix ocsigen/ocsigen.org-deprecated#19. To complete the fix, I will also remove ocamldoc markup from variands in lwt_unix.mli.

[Drup]

This looks good to me, but I certainly don't know enough of wikidoc's internals to be confident this patch is correct.

Ideally I would like to be able to test with ocsigen.org/ocsimore, but that's a bit delicate.

[aantron]

I can try setting up a local ocsigen.org for testing, following the instructions in its README, but probably early next week.