iis2mdc: add note about lis2mdl compatibility and link to datasheet

src/drivers/magnetometer/st/iis2mdc/iis2mdc.cpp

@@ -31,6 +31,10 @@
  *
  ****************************************************************************/
 
+// NOTE: this part is functionality equivalent to the lis2mdl
+// https://www.st.com/resource/en/design_tip/dt0131-digital-magnetometer-and-ecompass-efficient-design-tips--stmicroelectronics.pdf
+// https://www.st.com/resource/en/datasheet/iis2mdc.pdf
+
 #include "iis2mdc.h"
 
 using namespace time_literals;