[RDY] 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.
We should probably start a new paragraph here; it's very misleading, now that I read it again. E.g.:

```
The presence of a suitable clipboard tool implicitly enables the '+' and '*'
registers. 

If you want to ALWAYS use the clipboard for ALL operations (as opposed 
to interacting with the '+' and/or '*' registers explicitly), set the following option:
```