DOCS/lua: document the closed callback for mp.input.select

I wasn't sure if this was useful here but I ended up needing it in
select.lua. The other input.get callbacks still work, but are
undocumented because they are not useful for fuzzy finding.

DOCS/man/lua.rst

@@ -973,6 +973,9 @@ REPL.
         the 1-based index of the selected item. You can close the console from
         within the callback by calling ``input.terminate()``.
 
+    ``closed``
+        A callback invoked when the console is hidden.
+
     Example:
 
         ::