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
doc: Misc. improvements to nvim-related docs

- 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
Merge pull request #1624 from Pyrohh/doc-fixes

doc: Misc. improvements to nvim-related docs
deps: Update lua client
clipboard: support separate '+' and '*' clipboards
clipboard: handle linewise/charwise selections correctly
clipboard: clean up unnamedclip logic
options: change "unnamedclip" back to "clipboard=unnamed/unnamedplus"

This allows to configure which of '*' and '+' should be used for
the unnamed clipboard, and is consistent with vim.
clipboard: better error messages when provider not available
clipboard: add functional test
clipboard: fix `let @+ = ...` and add test
eval_has_provider: search autoload scripts same order as call_func