[RFC] Nvim specific docs: Misc. improvements

Besides the items mentioned in the commit message, most of the rewritten portions were done for clarity and consistency, but much of that is of course subjective so feel free to criticize.
It is customary in Vim helpdoc to reference options with `'foo'` instead of `|foo|`. Tags are created for both, and `''` is understood to mean an option rather than some other tag type.