CMSIS-DSP: Correct issue 888

Added clarifications to documentation about arm_power_...

Source/StatisticsFunctions/arm_power_f32.c

@@ -43,6 +43,10 @@
   </pre>
 
   There are separate functions for floating point, Q31, Q15, and Q7 data types.
+
+  Since the result is not divided by the length, those functions are in fact computing
+  something which is more an energy than a power.
+
  */
 
 /**