Mention serial port discovery in the docs

src/avrdude.1

@@ -610,6 +610,32 @@ serial port, the
 port is the default.  If you need to use a different parallel or
 serial port, use this option to specify the alternate port name.
 .Pp
+If
+.Nm
+has been configured with libserialport support, a serial port can be specified
+using a predefined serial adapter type in avrdude.conf or avrduderc, i. e.
+.Ar ch340
+or
+.Ar ft232r .
+If more than one serial adapter of the same type is connected, they can be
+distinguished by appending a serial number, i. e.
+.Ar ft232r:12345678 .
+Note that the USB to serial chip has to have a serial number for this to work.
+.Nm Avrdude
+can check for leading and trailing serial number matches as well.
+In the above example,
+.Ar ft232r:1234
+would also result in a match, and so would
+.Ar ft232r:...5678 .
+.Pp
+If the USB to serial chip is not known to
+.Nm ,
+it can be specified using the USB VID, PID and an optional serial number,
+following the serial number matching rules described above, e.g
+.Ar usb:0x2341:0x0043
+or
+.Ar usb:2341:0043:12345678 .
+.Pp
 On Win32 operating systems, the parallel ports are referred to as lpt1
 through lpt3, referring to the addresses 0x378, 0x278, and 0x3BC,
 respectively.  If the parallel port can be accessed through a different
@@ -984,7 +1010,6 @@ See below for a list of programmers accepting extended parameters
 or issue
 .Nm
 -x help ... to see the extended options of the chosen programmer.
-
 .El
 .Ss Terminal mode
 In this mode,

src/doc/avrdude.texi

@@ -373,7 +373,7 @@ Windows programming software.
 For many years, the AVRDUDE source resided in public repositories on
 savannah.nongnu.org,
 where it continued to be enhanced and ported to other systems.  In
-addition to FreeBSD, AVRDUDE now runs on Linux and Windows.  The
+addition to FreeBSD, AVRDUDE now runs on Linux, MacOS and Windows.  The
 developers behind the porting effort primarily were Ted Roth, Eric
 Weddington, and J@"org Wunsch.
 
@@ -687,6 +687,20 @@ used. See Appendix A, Platform Dependent Information, to find out the
 default port names for your platform. If you need to use a different
 parallel or serial port, use this option to specify the alternate port name.
 
+If AVRDUDE has been configured with libserialport support, a serial port can
+be specified using a predefined serial adapter type in @var{avrdude.conf} or
+@var{.avrduderc}, i. e. @code{ch340} or @code{ft232r}. If more than one serial
+adapter of the same type is connected, they can be distinguished by appending a
+serial number, i. e. @code{ft232r:12345678}. Note that the USB to serial chip
+has to have a serial number for this to work. AVRDUDE can check for leading and
+trailing serial number matches as well. In the above example, @code{ft232r:1234}
+would also result in a match, and so would @code{ft232r:...5678}.
+
+If the USB to serial chip is not known to AVRDUDE, it can be specified using
+the USB VID, PID and an optional serial number, following the serial number
+matching rules described above, e.g @code{usb:0x2341:0x0043} or
+@code{usb:2341:0043:12345678}.
+
 On Win32 operating systems, the parallel ports are referred to as lpt1
 through lpt3, referring to the addresses 0x378, 0x278, and 0x3BC,
 respectively.  If the parallel port can be accessed through a different