AP_Math: correct description of linear_interpolate

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

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

libraries/AP_Math/AP_Math.cpp

@@ -98,23 +98,23 @@ static void swap_float(float &f1, float &f2)
 /*
  * linear interpolation based on a variable in a range
  */
-float linear_interpolate(float low_output, float high_output,
-                         float var_value,
-                         float var_low, float var_high)
+float linear_interpolate(float output_low, float output_high,
+                         float input_value,
+                         float input_low, float input_high)
 {
-    if (var_low > var_high) {
+    if (input_low > input_high) {
         // support either polarity
-        swap_float(var_low, var_high);
-        swap_float(low_output, high_output);
+        swap_float(input_low, input_high);
+        swap_float(output_low, output_high);
     }
-    if (var_value <= var_low) {
-        return low_output;
+    if (input_value <= input_low) {
+        return output_low;
     }
-    if (var_value >= var_high) {
-        return high_output;
+    if (input_value >= input_high) {
+        return output_high;
     }
-    float p = (var_value - var_low) / (var_high - var_low);
-    return low_output + p * (high_output - low_output);
+    float p = (input_value - input_low) / (input_high - input_low);
+    return output_low + p * (output_high - output_low);
 }
 
 /* cubic "expo" curve generator

libraries/AP_Math/AP_Math.h

@@ -300,13 +300,18 @@ inline constexpr uint32_t usec_to_hz(uint32_t usec)
 
 /*
   linear interpolation based on a variable in a range
-  return value will be in the range [var_low,var_high]
+  return value will be in the range [output_low,output_high]
 
-  Either polarity is supported, so var_low can be higher than var_high
+  Either polarity is supported, so input_low can be higher than input_high
+
+  Read this as, "Take the value 'input_value' as a value between
+  'input_low' and 'input_high'.  Return a value between 'output_low'
+  and 'output_high' proportonal to the position of 'input_value'
+  between 'input_low' and 'input_high'"
  */
-float linear_interpolate(float low_output, float high_output,
-                         float var_value,
-                         float var_low, float var_high);
+float linear_interpolate(float output_low, float output_high,
+                         float input_value,
+                         float input_low, float input_high);
 
 /* cubic "expo" curve generator 
  * alpha range: [0,1] min to max expo