docs(undotree): rename help tag #36497

Problem: confusing that there is the tag `undo-tree` (the Vim
implementation) and `undotree` (the Lua plugin for visualization).

Solution: rename tag to undotree-plugin. Mention the plugin in the docs of
|undotree|.

src/gen/gen_vimdoc.lua

@@ -440,10 +440,13 @@ local config = {
       return 'Builtin plugin: ' .. name:lower()
     end,
     helptag_fmt = function(name)
-      if name:lower() == 'spellfile' then
+      name = name:lower()
+      if name == 'spellfile' then
         name = 'spellfile.lua'
+      elseif name == 'undotree' then
+        name = 'undotree-plugin'
       end
-      return name:lower()
+      return name
     end,
   },
 }