[FFT] added more documentation

src/shared/algorithms/FFT.h

@@ -24,6 +24,7 @@
 
 #include <complex.h>
 #include <math.h>
+
 #include <Eigen/Core>
 
 namespace Boardcore
@@ -105,10 +106,10 @@ public:
     }
 
 private:
-    static short n_bits(size_t x)
-    {
-        return (short)log2(x);
-    }
+    /**
+     * @brief Get the number of bits to differentiate x elements
+     */
+    static short n_bits(size_t x) { return (short)log2(x); }
 
     /**
      * @brief Reverse the bits of the inputted unsigned index.