docs(calendar): fix stale comments and integration docs

- diary.vim: correct calendar_action doc comment (dir is used for split
  direction; only week is unused)
- calendar-vim-integration.md: use real nuwiki var names
  (g:nuwiki_use_calendar / g:nuwiki_no_calendar / g:nuwiki_wikis), document
  auto-detect wiring and calendar-window close behavior; drop the
  non-existent refresh-on-close autocmd
- vimwiki-gap.md: remove stale 'no calendar.vim integration' entry now
  that the integration is implemented

autoload/nuwiki/diary.vim

@@ -5,7 +5,8 @@
 
 " Function: nuwiki#diary#calendar_action(day, month, year, week, dir)
 " Called by calendar-vim when user presses Enter on a date.
-" day/month/year are numeric integers. week and dir are not used.
+" day/month/year are numeric integers; dir is 'V' for a vertical split (any
+" other value means a horizontal split). week is unused.
 " Opens or creates the diary entry for the given date.
 function! nuwiki#diary#calendar_action(day, month, year, week, dir) abort
   " Build YYYY-MM-DD from the parts passed by calendar-vim