quad: update comment for hw_iface.h

quad/src/quad_app/hw_iface.h

@@ -13,6 +13,11 @@
  * hardware layer appropriate for your circumstance:
  * ../../xsdk_worksapce/real_quad -> running quad_app on the Zybo
  * ../virt_quad -> running quad_app in a Unix environment
+ *
+ * Function Pointer Return Values:
+ * - All driver functions return the error code
+ * - 0 for success
+ * - nonzero otherwise
  */
 
 // Forward declared types