runtime/doc: Misc. improvements

- Improved wording in a few places
- Refer references to Neovim as 'editor' to 'Nvim'
- Utilize vim's documentation features, e.g. unnamedclip -> |unnamedclip|
- Reflowed all changed paragraphs accordingly

- Various capitalization/grammar fixes
- Add spaces before parentheses
- Remove trailing whitespace
- Standardize single spaces after periods. Vim's docs use two for the
  most part, but Nvim's use one mainly, so just follow Nvim's conventions