Add a filament runout note (#19973)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/Configuration.h

@@ -1170,6 +1170,12 @@
  * Filament Runout Sensors
  * Mechanical or opto endstops are used to check for the presence of filament.
  *
+ * IMPORTANT: Runout will only trigger if Marlin is aware that a print job is running.
+ * Marlin knows a print job is running when:
+ *  1. Running a print job from media started with M24.
+ *  2. The Print Job Timer has been started with M75.
+ *  3. The heaters were turned on and PRINTJOB_TIMER_AUTOSTART is enabled.
+ *
  * RAMPS-based boards use SERVO3_PIN for the first runout sensor.
  * For other boards you may need to define FIL_RUNOUT_PIN, FIL_RUNOUT2_PIN, etc.
  */