[wpimath] Improve documentation for SwerveModulePosition::operator- (#5468)

wpimath/src/main/native/include/frc/kinematics/SwerveModulePosition.h

@@ -35,6 +35,16 @@ struct WPILIB_DLLEXPORT SwerveModulePosition {
    */
   bool operator==(const SwerveModulePosition& other) const;
 
+  /**
+   * Calculates the difference between two swerve module positions. The
+   * difference has a length equal to the difference in lengths and an angle
+   * equal to the ending angle (this module position's angle). This is suitable
+   * for representing a module's motion between two timesteps, because the final
+   * angle describes the direction the module moved.
+   *
+   * @param other The other swerve module position to subtract.
+   * @return The difference.
+   */
   SwerveModulePosition operator-(const SwerveModulePosition& other) const {
     return {distance - other.distance, angle};
   }