Document homeaxis

Marlin/Marlin_main.cpp

   endstops.hit_on_purpose();
 }
 
+/**
+ * Home an individual "raw axis" to its endstop.
+ * This applies to XYZ on Cartesian and Core robots, and
+ * to the individual ABC steppers on DELTA and SCARA.
+ *
+ * At the end of the procedure the axis is marked as
+ * homed and the current position of that axis is updated.
+ * Kinematic robots should wait till all axes are homed
+ * before updating the current position.
+ */
+
 #define HOMEAXIS(LETTER) homeaxis(LETTER##_AXIS)
 
 static void homeaxis(AxisEnum axis) {