runtime/docs/msgpack-rpc: Misc. fixes

- Correct apostrophe usage, primarily with it's/its
- Correct usage of 'e.g.'
- Capitalize acronyms
- Prepend NVIM_LISTEN_ADDRESS with '$' to match previous instance
- Avoid awkward phrasing
- Make lists with an explicitly stated number of points numbered to
  match other such lists
- Added space before parentheses
- rm traling whitespace