DOCS/man/lua.rst: document custom arg for mp.add_key_binding()

DOCS/man/lua.rst

@@ -316,8 +316,8 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     or pass the ``fn`` argument in place of the name. The latter is not
     recommended and is handled for compatibility only.)
 
-    The last argument is used for optional flags. This is a table, which can
-    have the following entries:
+    The ``flags`` argument is used for optional parameters. This is a table,
+    which can have the following entries:
 
         ``repeatable``
             If set to ``true``, enables key repeat for this specific binding.
@@ -358,6 +358,11 @@ The ``mp`` module is preloaded, although it can be loaded manually with
                     The scale of the key, such as the ones produced by ``WHEEL_*``
                     keys. The scale is 1 if the key is nonscalable.
 
+                ``arg``
+                    User-provided string in the ``arg`` argument in the
+                    ``script-binding`` command if the key binding is invoked
+                    by that command.
+
     Internally, key bindings are dispatched via the ``script-message-to`` or
     ``script-binding`` input commands and ``mp.register_script_message``.