Document reason for include

Author: niklasf

Modules/mathmodule.c

@@ -53,7 +53,7 @@ raised for division by zero and mod by zero.
  */
 
 #include "Python.h"
-#include "pycore_bitutils.h"
+#include "pycore_bitutils.h" // _Py_bit_length()
 #include "pycore_dtoa.h"
 #include "_math.h"