Improve and document the language-change event on MS-Windows.

Improve and document the language-change event on MS-Windows.

 src/keyboard.c (kbd_buffer_get_event): Include the codepage and the
 language ID in the event parameters.
 src/w32term.c (w32_read_socket): Put the new keyboard codepage into
 event.code, not the obscure "character set ID".
 doc/lispref/commands.texi (Misc Events): Document the language-change event.