Include a comment documenting M110

Marlin/Marlin_main.cpp

  * M109 - Sxxx Wait for extruder current temp to reach target temp. Waits only when heating
  *        Rxxx Wait for extruder current temp to reach target temp. Waits when heating and cooling
  *        IF AUTOTEMP is enabled, S<mintemp> B<maxtemp> F<factor>. Exit autotemp by any M109 without F
+ * M110 - Set the current line number
  * M111 - Set debug flags with S<mask>. See flag bits defined in Marlin.h.
  * M112 - Emergency stop
  * M114 - Output current position to serial port