docs: add lua typing for vim.NIL

runtime/lua/vim/_meta/builtin.lua

@@ -1,7 +1,8 @@
 ---@meta
-
 -- luacheck: no unused args
 
+error('Cannot require a meta file')
+
 ---@defgroup vim.builtin
 ---
 ---@brief <pre>help
@@ -62,6 +63,12 @@
 ---
 ---</pre>
 
+---@class vim.NIL
+
+---@type vim.NIL
+---@nodoc
+vim.NIL = ...
+
 --- Returns true if the code is executing as part of a "fast" event handler,
 --- where most of the API is disabled. These are low-level events (e.g.
 --- |lua-loop-callbacks|) which can be invoked whenever Nvim polls for input.

runtime/lua/vim/_meta/vvars.lua

@@ -427,7 +427,7 @@ vim.v.msgpack_types = ...
 --- In some places `v:null` can be used for a List, Dict, etc.
 --- that is not set.  That is slightly different than an empty
 --- List, Dict, etc.
---- @type any
+--- @type vim.NIL
 vim.v.null = ...
 
 --- Maximum value of a number.

runtime/lua/vim/lsp/util.lua

@@ -2139,7 +2139,7 @@ end
 ---
 ---@param settings table language server settings
 ---@param section  string indicating the field of the settings table
----@return table|string The value of settings accessed via section
+---@return table|string|vim.NIL The value of settings accessed via section. `vim.NIL` if not found.
 function M.lookup_section(settings, section)
   for part in vim.gsplit(section, '.', { plain = true }) do
     settings = settings[part]