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

DOCS/man/lua.rst

@@ -284,7 +284,7 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     Return the current mpv internal time in seconds as a number. This is
     basically the system time, with an arbitrary offset.
 
-``mp.add_key_binding(key, name|fn [,fn [,flags]])``
+``mp.add_key_binding(key, name|fn [,fn [,flags, [arg]]])``
     Register callback to be run on a key binding. The binding will be mapped to
     the given ``key``, which is a string describing the physical key. This uses
     the same key names as in input.conf, and also allows combinations
@@ -316,7 +316,7 @@ 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
+    The last argument is used for optional parameters. This is a table, which can
     have the following entries:
 
         ``repeatable``
@@ -327,6 +327,11 @@ The ``mp`` module is preloaded, although it can be loaded manually with
             If set to ``true``, enables key scaling for this specific binding.
             This option only makes sense when ``complex`` is set to ``true``.
 
+        ``arg``
+            A user-provided string which is available in the table argument of
+            ``fn`` by default.
+            This option only makes sense when ``complex`` is set to ``true``.
+
         ``complex``
             If set to ``true``, then ``fn`` is called on key down, repeat and up
             events, with the first argument being a table. This table has the
@@ -358,6 +363,11 @@ 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, or the ``arg``
+                    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``.