[1.1.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-Remove outdated 256 size limitation from comments on RX_BUFFER_SIZE defines
config updates
-Update error checking conditions

Marlin/Marlin_main.cpp

@@ -8490,6 +8490,13 @@ inline void gcode_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");