use bufnr instead of winnr to make sure to identify the info window
improve document for command, mapping and dependency

- more detail about :GrammarousCheck
- <Plug>(grammarous-close-info-window)
- more dependency (vimproc)
enable to specify if use fallback or not
improve message to report grammatical errors
improve error messages
tiny fixes

- code
  - enable grammarous#close_info_window() anywhere
  - don't do auto preview on visual mode
  - use augroup for the autocmd
  - Capitalize message
  - fix defining b:grammarous_preview_bufnr
- doc
  - fix title
  -
  - trivial fixes
implement the feature to remove an error
remove the error when it is fixed automatically
show help with '?' in the info window