Merge pull request #6689 from mali/fix_6631

doc/saul: add cross link between SAUL and SAUL registry

drivers/include/saul.h

@@ -31,6 +31,8 @@
  *              operations. It probably needs to be extended to handling events,
  *              thresholds, and so on.
  *
+ * @see @ref sys_saul_reg
+ *
  * @{
  *
  * @file

sys/include/saul_reg.h

@@ -11,6 +11,8 @@
  * @ingroup     sys
  * @brief       Global sensor/actuator registry for SAUL devices
  *
+ * @see @ref drivers_saul
+ *
  * @{
  *
  * @file