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: #16655

Co-authored-by: dkearns <dougkearns@gmail.com>
Signed-off-by: brianhuster <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/filetype.txt

@@ -1,4 +1,4 @@
-*filetype.txt*	For Vim version 9.1.  Last change: 2025 Feb 20
+*filetype.txt*	For Vim version 9.1.  Last change: 2025 Feb 25
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -744,12 +744,21 @@ Add following lines to $HOME/.vim/ftplugin/json.vim: >
 	import autoload 'dist/json.vim'
 	setl formatexpr=json.FormatExpr()
 
-LUA							*ft-lua-plugin*
+LUA					*ft-lua-plugin* *g:lua_folding*
 
 You can enable folding of Lua functions using |fold-expr| by: >
 
 	let g:lua_folding = 1
+<					*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

@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2025 Feb 23
+*syntax.txt*	For Vim version 9.1.  Last change: 2025 Feb 25
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2343,13 +2343,9 @@ 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*

runtime/doc/tags

@@ -7625,6 +7625,9 @@ g:html_use_input_for_pc	syntax.txt	/*g:html_use_input_for_pc*
 g:html_use_xhtml	syntax.txt	/*g:html_use_xhtml*
 g:html_whole_filler	syntax.txt	/*g:html_whole_filler*
 g:lf_shell_syntax	syntax.txt	/*g:lf_shell_syntax*
+g:lua_folding	filetype.txt	/*g:lua_folding*
+g:lua_subversion	filetype.txt	/*g:lua_subversion*
+g:lua_version	filetype.txt	/*g:lua_version*
 g:markdown_fenced_languages	syntax.txt	/*g:markdown_fenced_languages*
 g:markdown_minlines	syntax.txt	/*g:markdown_minlines*
 g:markdown_syntax_conceal	syntax.txt	/*g:markdown_syntax_conceal*