AP_Math: correct description of linear_interpolate

[peterbarker]

the return-value comment was just flat-out wrong.

Fix the parameter naming to make it clearer what is going on

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
CubeRedPrimary                      *      *           *       *                 *      *      *
Durandal                            *      *           *       *                 *      *      *
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     *      *           *       *                 *      *      *
MatekF405                           *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot                  *                  *       *                 *      *      *
bebop                               *                  *       *                 *      *      *
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           *      *           *       *                 *      *      *
skyviper-v2450                                         *