[LuaDoc] Document ndirs member of a directory listing

utils/emmylua/filesystem.lua

@@ -2,12 +2,15 @@
 
 filesystem = {}
 
+---@class directory_listing : string[]
+---@field ndirs integer
+
 ---Read a text file into memory, or list files in a directory
 ---@param path string
----@return string|string[]
+---@return string|directory_listing
 function filesystem.read_file(path) end
 
----Check if a file exists
+---Check if a file exists and whether
 ---@param path string
 ---@param real_file? boolean If true, the file must be a real file and not for example a directory.
 ---@return boolean