Fix lua_language_server_config variable name (#5025)

Since added in commit f2a21c9, the implementation and documentation has
differed in their naming. The latter has had an additional lua_-prefix.

With this fix, setting the variable as documented actually results in a
'workspace/didChangeConfiguration' method being executed on the luals.

Adding a reference to the documentation of which configurations the lsp
supports doesn't hurt, so that's done too.

Co-authored-by: cos <cos>

doc/ale-lua.txt

@@ -49,17 +49,22 @@ g:ale_lua_language_server_executable
   If you have compiled the language server yourself in `/some/path`, the path
   will be `'/some/path/bin/lua-language-server'`.
 
-                                   *ale-options.lua_lua_language_server_config*
-                                         *g:ale_lua_lua_language_server_config*
-                                         *b:ale_lua_lua_language_server_config*
-lua_lua_language_server_config
-g:ale_lua_lua_language_server_config
+                                       *ale-options.lua_language_server_config*
+                                             *g:ale_lua_language_server_config*
+                                             *b:ale_lua_language_server_config*
+lua_language_server_config
+g:ale_lua_language_server_config
+
   Type: |Dictionary|
   Default: `{}`
 
   Dictionary containing configuration settings that will be passed to the
   language server.
 
+  For available configuration parameters, see `Settings` on the luals wiki:
+
+  https://luals.github.io/wiki/settings/
+
 
 ===============================================================================
 luac                                                             *ale-lua-luac*