[2.0.x] Report SERIAL_XON_XOFF in M115

-Report SERIAL_XON_XOFF in Extended Capabilites M115 report for hosts
-Remove outdated 256 size limitation from comments on RX_BUFFER_SIZE defines
-Update error message conditions

Marlin/src/gcode/host/M115.cpp

@@ -31,6 +31,13 @@ void GcodeSuite::M115() {
 
   #if ENABLED(EXTENDED_CAPABILITIES_REPORT)
 
+    // SERIAL_XON_XOFF
+    #if ENABLED(SERIAL_XON_XOFF)
+      SERIAL_PROTOCOLLNPGM("Cap:SERIAL_XON_XOFF:1");
+    #else
+      SERIAL_PROTOCOLLNPGM("Cap:SERIAL_XON_XOFF:0");
+    #endif
+
     // EEPROM (M500, M501)
     #if ENABLED(EEPROM_SETTINGS)
       SERIAL_PROTOCOLLNPGM("Cap:EEPROM:1");