Add documentation about serial port access.
\f
* Changes in Emacs 23.1
+** Emacs now supports serial port access on GNU/Linux, Unix, and
+Windows. `serial-term' starts an interactive terminal,
+`make-serial-process' and `serial-process-configure' provide a Lisp
+interface. The serial port can be configured at runtime with the
+mode-line mouse menu.
+
** Apropos
*** `apropos-library' describes the elements defined in a given library.
*** Set `apropos-compact-layout' is you want a more compact (but wider) layout.