[docs] Add missing JavaDocs (#6146)

wpimath/src/main/java/edu/wpi/first/math/interpolation/InterpolatingDoubleTreeMap.java

@@ -9,6 +9,7 @@ package edu.wpi.first.math.interpolation;
  * from points that are defined. This uses linear interpolation.
  */
 public class InterpolatingDoubleTreeMap extends InterpolatingTreeMap<Double, Double> {
+  /** Default constructor. */
   public InterpolatingDoubleTreeMap() {
     super(InverseInterpolator.forDouble(), Interpolator.forDouble());
   }

wpimath/src/main/java/edu/wpi/first/math/interpolation/Interpolator.java

@@ -24,6 +24,11 @@ public interface Interpolator<T> {
    */
   T interpolate(T startValue, T endValue, double t);
 
+  /**
+   * Returns interpolator for Double.
+   *
+   * @return Interpolator for Double.
+   */
   static Interpolator<Double> forDouble() {
     return MathUtil::interpolate;
   }

wpimath/src/main/java/edu/wpi/first/math/interpolation/InverseInterpolator.java

@@ -24,6 +24,11 @@ public interface InverseInterpolator<T> {
    */
   double inverseInterpolate(T startValue, T endValue, T q);
 
+  /**
+   * Returns inverse interpolator for Double.
+   *
+   * @return Inverse interpolator for Double.
+   */
   static InverseInterpolator<Double> forDouble() {
     return MathUtil::inverseInterpolate;
   }