Document set_axis_is_at_home

Marlin/Marlin_main.cpp

@@ -1323,6 +1323,23 @@ static void set_home_offset(AxisEnum axis, float v) {
   update_software_endstops(axis);
 }
 
+/**
+ * Set an axis' current position to its home position (after homing).
+ *
+ * For Core and Cartesian robots this applies one-to-one when an
+ * individual axis has been homed.
+ *
+ * DELTA should wait until all homing is done before setting the XYZ
+ * current_position to home, because homing is a single operation.
+ * In the case where the axis positions are already known and previously
+ * homed, DELTA could home to X or Y individually by moving either one
+ * to the center. However, homing Z always homes XY and Z.
+ *
+ * SCARA should wait until all XY homing is done before setting the XY
+ * current_position to home, because neither X nor Y is at home until
+ * both are at home. Z can however be homed individually.
+ * 
+ */
 static void set_axis_is_at_home(AxisEnum axis) {
   #if ENABLED(DEBUG_LEVELING_FEATURE)
     if (DEBUGGING(LEVELING)) {