fix(doc): remove duplicate g:nuwiki_no_calendar tag breaking helptags

`*g:nuwiki_no_calendar*` was defined twice, so `:helptags` aborted with
E154 and generated no tags at all — plugin managers surface this as an
error on every install/update.

The copy on the Lua `use_calendar` entry couldn't be the definition: its
own prose links to |g:nuwiki_no_calendar|. Dropped it, made the
Vim-specific globals entry canonical (right-aligned like its neighbours),
and gave it the description it was missing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nur6RmP49J1o6agftanwLR

doc/nuwiki.txt

@@ -187,7 +187,6 @@ plugin: >
     pages. Honours `links_space_char` (so `My_Page.wiki` → `= My Page =`).
     Mirrors vimwiki's `auto_header`. Vim users set |g:nuwiki_auto_header|.
 
-                                                         *g:nuwiki_no_calendar*
 `use_calendar`     true
     When true (default), wire up calendar-vim integration if it is on the
     runtimepath (`g:calendar_action` / `g:calendar_sign`). Set to false, or
@@ -380,6 +379,8 @@ Vim-specific globals ~
 
                                                         *g:nuwiki_no_calendar*
 `g:nuwiki_no_calendar`         0
+    When 1, skip the calendar-vim integration entirely. The Vim
+    equivalent of setting `use_calendar = false` in Neovim's setup().
 
                                                           *g:nuwiki_binary_path*
 `g:nuwiki_binary_path`