sys/auto_init: Improved doc on sht1x auto init

The auto initialization of the sht1x module differs from the initialization of
other sensors, but previously no documentation pointed that out. This lack of
documentation led to a bug being introduced. This commit provides the previously
missing documentation.

sys/auto_init/auto_init.c

@@ -296,6 +296,10 @@ void auto_init(void)
 
 /* initialize sensors and actuators */
 #ifdef MODULE_SHT1X
+    /* The sht1x module needs to be initialized regardless of SAUL being used,
+     * as the shell commands rely on auto-initialization. auto_init_sht1x also
+     * performs SAUL registration, but only if module auto_init_saul is used.
+     */
     DEBUG("Auto init SHT1X module (SHT10/SHT11/SHT15 sensor driver).\n");
     extern void auto_init_sht1x(void);
     auto_init_sht1x();

sys/auto_init/saul/auto_init_sht1x.c

@@ -16,6 +16,11 @@
  *
  * @author      Marian Buschsieweke <marian.buschsieweke@ovgu.de>
  *
+ * Please note that the sht1x differs from other sensor modules, as
+ * initialization is performed regardless of the use of SAUL. However, when
+ * the module `auto_init_saul` is *not* used, the SAUL registration is skipped
+ * and the initialization does *not* depend on SAUL in this case.
+ *
  * @}
  */