[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.
#1182 removes `'unnamedclip'`, because it's redundant with `clipboard=unnamedplus`. Let's change this entire paragraph to:

```
The presence of a suitable clipboard tool implicitly enables the '+' and '*' registers.
To use the clipboard for all operations:
>
    set clipboard+=unnamedplus
<
```