Fix M118 parameters, with strict guideline

Marlin/Marlin_main.cpp

 /**
  * M118: Display a message in the host console.
  *
- *  A  Append '// ' for an action command, as in OctoPrint
- *  E  Have the host 'echo:' the text
+ *  A1  Append '// ' for an action command, as in OctoPrint
+ *  E1  Have the host 'echo:' the text
  */
 inline void gcode_M118() {
   if (parser.boolval('E')) SERIAL_ECHO_START();

Marlin/gcode.cpp

   #endif
 
   // Only use string_arg for these M codes
-  if (letter == 'M') switch (codenum) { case 23: case 28: case 30: case 117: case 118: case 928: string_arg = p; return; default: break; }
+  if (letter == 'M') switch (codenum) { case 23: case 28: case 30: case 117: case 928: string_arg = p; return; default: break; }
 
   #if ENABLED(DEBUG_GCODE_PARSER)
     const bool debug = codenum == 800;
    * Most codes ignore 'string_arg', but those that want a string will get the right pointer.
    * The following loop assigns the first "parameter" having no numeric value to 'string_arg'.
    * This allows M0/M1 with expire time to work: "M0 S5 You Win!"
+   * For 'M118' you must use 'E1' and 'A1' rather than just 'E' or 'A'
    */
   string_arg = NULL;
   while (char code = *p++) {                    // Get the next parameter. A NUL ends the loop