[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-Remove outdated 256 size limitation from comments on RX_BUFFER_SIZE defines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/HAL/HAL_DUE/MarlinSerial_Due.h

@@ -64,7 +64,7 @@
 #endif
 
 #if TX_BUFFER_SIZE && (TX_BUFFER_SIZE < 2 || TX_BUFFER_SIZE > 256 || !IS_POWER_OF_2(TX_BUFFER_SIZE))
-  #error "TX_BUFFER_SIZE must be 0 or a power of 2 greater than 1."
+  #error "TX_BUFFER_SIZE must be 0, a power of 2 greater than 1, and no greater than 256."
 #endif
 
 #if RX_BUFFER_SIZE > 256

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");

Marlin/src/inc/Conditionals_adv.h

@@ -30,10 +30,12 @@
 
   #ifndef USBCON
     // Define constants and variables for buffering incoming serial data.
-    // 256 is the max limit due to uint8_t head and tail. Use only powers of 2. (...,16,32,64,128,256)
+    // Use only powers of 2.
+    // : [0, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, ...]
     #ifndef RX_BUFFER_SIZE
       #define RX_BUFFER_SIZE 128
     #endif
+    // 256 is the max TX buffer climit due to uint8_t head and tail.
     #ifndef TX_BUFFER_SIZE
       #define TX_BUFFER_SIZE 32
     #endif

Marlin/src/inc/SanityCheck.h

@@ -262,7 +262,7 @@
 
   // 256 is the max limit due to uint8_t head and tail. Use only powers of 2. (...,16,32,64,128,256)
   #if TX_BUFFER_SIZE && (TX_BUFFER_SIZE < 2 || TX_BUFFER_SIZE > 256 || !IS_POWER_OF_2(TX_BUFFER_SIZE))
-    #error "TX_BUFFER_SIZE must be 0 or a power of 2 greater than 1."
+    #error "TX_BUFFER_SIZE must be 0, a power of 2 greater than 1, and no greater than 256."
   #endif
 #endif