doc/wrs-user-manual: added description of wrs_port_tx_control

Signed-off-by: Tjeerd Pinkert <t.j.pinkert@alumnus.utwente.nl>

doc/wrs-user-manual.in

@@ -1145,6 +1145,14 @@ The following tools and scripts are provided:
         of the PPSi process, but PPSi switches the PPS output back on when a
         link restart is detected and PPSi comes into @t{'TRACK_PHASE'} state.
 
+@item wrs_port_tx_control
+        A tool to switch the TX (laser) of a port on and off by use of the @t{TX
+        disable} pin on the SFP. Usage ``@t{wrs_port_tx_control <port_no> on}''
+        switches the TX on, and ``@t{wrs_port_tx_control <port_no> off}''
+        switches the TX off. The port number @t{<port_no>} ranges from 0 to the
+        maximum number of ports on the device minus one, and is currently
+        limited to 17.
+
 @item wr_date
 
 	The program can read or set the White Rabbit date. When setting,