AP_SerialManager: added note on external converter requirement for Frsky

thanks to Luis for the suggestion

libraries/AP_SerialManager/AP_SerialManager.cpp

@@ -36,7 +36,7 @@ const AP_Param::GroupInfo AP_SerialManager::var_info[] PROGMEM = {
 
     // @Param: 1_PROTOCOL
     // @DisplayName: Telem1 protocol selection
-    // @Description: Control what protocol to use on the Telem1 port
+    // @Description: Control what protocol to use on the Telem1 port. Note that the Frsky options require external converter hardware. See the wiki for details.
     // @Values: 1:GCS Mavlink, 2:GCS Mavlink (2nd), 3:Frsky D-PORT, 4:Frsky S-PORT, 5:GPS, 6:2nd GPS, 7:Alexmos Gimbal Serial
     // @User: Standard
     AP_GROUPINFO("1_PROTOCOL",  1, AP_SerialManager, state[1].protocol, SerialProtocol_MAVLink1),
@@ -50,7 +50,7 @@ const AP_Param::GroupInfo AP_SerialManager::var_info[] PROGMEM = {
 
     // @Param: 2_PROTOCOL
     // @DisplayName: Telemetry 2 protocol selection
-    // @Description: Control what protocol to use on the Telem2 port
+    // @Description: Control what protocol to use on the Telem2 port. Note that the Frsky options require external converter hardware. See the wiki for details.
     // @Values: 1:GCS Mavlink, 2:GCS Mavlink (2nd), 3:Frsky D-PORT, 4:Frsky S-PORT, 5:GPS, 6:GPS (2nd), 7:Alexmos Gimbal Serial
     // @User: Standard
     AP_GROUPINFO("2_PROTOCOL",  3, AP_SerialManager, state[2].protocol, SerialProtocol_MAVLink2),
@@ -64,7 +64,7 @@ const AP_Param::GroupInfo AP_SerialManager::var_info[] PROGMEM = {
 
     // @Param: 3_PROTOCOL
     // @DisplayName: Serial 3 (GPS) protocol selection
-    // @Description: Control what protocol Serial 3 (GPS) should be used for
+    // @Description: Control what protocol Serial 3 (GPS) should be used for. Note that the Frsky options require external converter hardware. See the wiki for details.
     // @Values: 1:GCS Mavlink, 2:GCS Mavlink (2nd), 3:Frsky D-PORT, 4:Frsky S-PORT, 5:GPS, 6:GPS (2nd), 7:Alexmos Gimbal Serial
     // @User: Standard
     AP_GROUPINFO("3_PROTOCOL",  5, AP_SerialManager, state[3].protocol, SerialProtocol_GPS),
@@ -78,7 +78,7 @@ const AP_Param::GroupInfo AP_SerialManager::var_info[] PROGMEM = {
 
     // @Param: 4_PROTOCOL
     // @DisplayName: Serial4 protocol selection
-    // @Description: Control what protocol Serial4 port should be used for
+    // @Description: Control what protocol Serial4 port should be used for. Note that the Frsky options require external converter hardware. See the wiki for details.
     // @Values: 1:GCS Mavlink, 2:GCS Mavlink (2nd), 3:Frsky D-PORT, 4:Frsky S-PORT, 5:GPS, 6:GPS (2nd), 7:Alexmos Gimbal Serial
     // @User: Standard
     AP_GROUPINFO("4_PROTOCOL",  7, AP_SerialManager, state[4].protocol, SerialProtocol_GPS2),