Adding separate RX_BUFFER_SIZE configuration setting and proper documentation on its usage

Marlin/Configuration_adv.h

 #define MAX_CMD_SIZE 96
 #define BUFSIZE 4
 
-// Transfer Buffer Size
+// Transmission to Host Buffer Size
 // To save 386 bytes of PROGMEM (and TX_BUFFER_SIZE+3 bytes of RAM) set to 0.
 // To buffer a simple "ok" you need 4 bytes.
 // For ADVANCED_OK (M105) you need 32 bytes.
 // :[0, 2, 4, 8, 16, 32, 64, 128, 256]
 #define TX_BUFFER_SIZE 0
 
+// Reception from Host Buffer Size
+// This is the size of the Reception buffer. If XON/XOFF software flow control
+// is not enabled, then 32 bytes should be enough. But if you plan to use XON/XOFF
+// you need 1024 bytes at least.
+// :[0, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048]
+#define RX_BUFFER_SIZE 1024
+
 // Enable an emergency-command parser to intercept certain commands as they
 // enter the serial receive buffer, so they cannot be blocked.
 // Currently handles M108, M112, M410