Add some warnings in lcd_buttons_update

Firmware/lcd.cpp

@@ -730,6 +730,8 @@ void lcd_update_enable(uint8_t enabled)
 
 bool lcd_longpress_trigger = 0;
 
+// WARNING: this function is called from the temperature ISR.
+//          Only update flags, but do not perform any menu/lcd operation!
 void lcd_buttons_update(void)
 {
     static uint8_t lcd_long_press_active = 0;