docs: Remove shortname references.

In addition, align the removal notice next to 'key' with other
such notices (options.txt).
runtime: Cleanup refs to removed options

- {no,inv,}shortname
- {no,inv,}sn
- cm, key (remnants of cryptmethod)
Merge pull request #1623 from Pyrohh/cleanup-shortname

Docs: Remove 'shortname' references
Nvim specific docs: Misc. improvements

- Improved wording in a few places for clarity
- Various capitalization/grammar fixes
- Change references to Neovim as 'editor' to 'Nvim'
- Be consistent regarding utilization of vim's documentation features,
  e.g. unnamedclip -> |unnamedclip|
- Reflowed all changed paragraphs accordingly

- 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