vim-patch:00a00f5: runtime(lua): Update lua ftplugin and documentation

Problem:
- The doc says the default `g:lua_subversion` is 2, but in fact it is 3
  (see `runtime/syntax/lua.vim`)
- `includeexpr` doesn't work with module in `init.lua`

Solution:
- Update documentation
- Assign value to option `&include`
- Add function `LuaInclude` and assign it to `l:&includeexpr`

closes: vim/vim#16655

00a00f5d3f

Co-authored-by: brianhuster <phambinhanctb2004@gmail.com>
Co-authored-by: dkearns <dougkearns@gmail.com>

runtime/doc/filetype.txt

@@ -695,7 +695,18 @@ To enable the recognition of Markdown comments each time after removing
 re-source "javaformat.vim" for Vim versions greater than `8.2.1397`: >
 	runtime autoload/javaformat.vim
 <
+LUA							*ft-lua-plugin*
 
+					*g:lua_version* *g:lua_subversion*
+Lua filetype's 'includeexpr' and |ft-lua-syntax| highlighting use the global
+variables "g:lua_version" and "g:lua_subversion" to determine the version of
+Lua to use (5.3 is the default)
+
+For example, to use Lua 5.1, set the variables like this: >
+
+	let g:lua_version = 5
+	let g:lua_subversion = 1
+<
 MAIL							*ft-mail-plugin*
 
 Options:

runtime/doc/syntax.txt

@@ -1870,13 +1870,9 @@ instead, and the name of your source file should be `*.pike`
 
 LUA						*lua.vim* *ft-lua-syntax*
 
-The Lua syntax file can be used for versions 4.0, 5.0, 5.1 and 5.2 (5.2 is
-the default). You can select one of these versions using the global variables
-lua_version and lua_subversion. For example, to activate Lua
-5.1 syntax highlighting, set the variables like this: >
-
-	:let lua_version = 5
-	:let lua_subversion = 1
+The Lua syntax file can be used for versions 4.0, 5.0+. You can select one of
+these versions using the global variables |g:lua_version| and
+|g:lua_subversion|.
 
 
 MAIL						*mail.vim* *ft-mail.vim*