speakup: Document USB support

Speakup has been supporting USB for a while already.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231020181059.7rtj2csi7t6vorrm@begin

Documentation/admin-guide/spkguide.txt

@@ -7,7 +7,7 @@ Last modified on Mon Sep 27 14:26:31 2010
 Document version 1.3
 
 Copyright (c) 2005  Gene Collins
-Copyright (c) 2008  Samuel Thibault
+Copyright (c) 2008, 2023  Samuel Thibault
 Copyright (c) 2009, 2010  the Speakup Team
 
 Permission is granted to copy, distribute and/or modify this document
@@ -83,8 +83,7 @@ spkout -- Speak Out
 txprt -- Transport
 dummy -- Plain text terminal
 
-Note: Speakup does * NOT * support usb connections!  Speakup also does *
-NOT * support the internal Tripletalk!
+Note: Speakup does * NOT * support the internal Tripletalk!
 
 Speakup does support two other synthesizers, but because they work in
 conjunction with other software, they must be loaded as modules after
@@ -94,6 +93,12 @@ These are as follows:
 decpc -- DecTalk PC (not available at boot up)
 soft -- One of several software synthesizers (not available at boot up)
 
+By default speakup looks for the synthesizer on the ttyS0 serial port. This can
+be changed with the device parameter of the modules, for instance for
+DoubleTalk LT:
+
+speakup_ltlk.dev=ttyUSB0
+
 See the sections on loading modules and software synthesizers later in
 this manual for further details.  It should be noted here that the
 speakup.synth boot parameter will have no effect if Speakup has been