pbio/int_math: Document truncation for mult_then_div.

This can lead to small (non-accumalative) rounding errors in user-getter functions, like robot.angle(). This is generally not an issue, but helpful to be aware of.

Author: laurensvalk

lib/pbio/src/angle.c

@@ -132,7 +132,7 @@ int32_t pbio_angle_to_low_res(const pbio_angle_t *a, int32_t scale) {
         return 0;
     }
 
-    // Scale down rotations component.
+    // Scale down rotations component. NB: Truncates, does not round.
     int32_t rotations_component = pbio_int_math_mult_then_div(a->rotations, MDEG_PER_ROT, scale);
 
     // Scale down millidegree component, rounded to nearest ouput unit.

lib/pbio/src/int_math.c

@@ -257,6 +257,8 @@ int32_t pbio_int_math_atan2(int32_t y, int32_t x) {
  * The product of @p a and @p b must not exceed 2**47, and the result after
  * division must not exceed 2**31. @p c must not exceed 2**16.
  *
+ * Truncates the result as conventional with integer division.
+ *
  * Adapted from https://stackoverflow.com/a/57727180, CC BY-SA 4.0
  *
  * @param [in]  a    Positive or negative number.