Add a note about substitution chars

Marlin/src/lcd/language/language_en.h

@@ -26,6 +26,13 @@
  *
  * LCD Menu Messages
  * See also https://marlinfw.org/docs/development/lcd_language.html
+ *
+ * Substitutions are applied for the following characters when used
+ * in menu items that call lcd_put_u8str_ind_P with an index:
+ *
+ *   = displays  '0'....'10' for indexes 0 - 10
+ *   ~ displays  '1'....'11' for indexes 0 - 10
+ *   * displays 'E1'...'E11' for indexes 0 - 10 (By default. Uses LCD_FIRST_TOOL)
  */
 
 #define en 1234