Update tool which generates lua module docs

All "helper functions" are not part of the Lua code for module pandoc,
but are added in Haskell. The respective documentation section must
therefore be excluded from automatic regeneration.

tools/update-lua-docs.lua

@@ -25,7 +25,9 @@ end
 
 function Header (el)
   if in_module_section then
-    if el.level == 1 then
+    if el.level == 1 or
+       -- special case for Module pandoc
+       (el.level == 2 and el.identifier == 'helper-functions') then
       in_module_section = false
       return el
     else