[docs] Add missing JavaDocs (#6146)

wpimath/src/main/java/edu/wpi/first/math/spline/CubicHermiteSpline.java

@@ -6,6 +6,7 @@ package edu.wpi.first.math.spline;
 
 import org.ejml.simple.SimpleMatrix;
 
+/** Represents a hermite spline of degree 3. */
 public class CubicHermiteSpline extends Spline {
   private static SimpleMatrix hermiteBasis;
   private final SimpleMatrix m_coefficients;

wpimath/src/main/java/edu/wpi/first/math/spline/PoseWithCurvature.java

@@ -8,10 +8,10 @@ import edu.wpi.first.math.geometry.Pose2d;
 
 /** Represents a pair of a pose and a curvature. */
 public class PoseWithCurvature {
-  // Represents the pose.
+  /** Represents the pose. */
   public Pose2d poseMeters;
 
-  // Represents the curvature.
+  /** Represents the curvature. */
   public double curvatureRadPerMeter;
 
   /**

wpimath/src/main/java/edu/wpi/first/math/spline/QuinticHermiteSpline.java

@@ -6,6 +6,7 @@ package edu.wpi.first.math.spline;
 
 import org.ejml.simple.SimpleMatrix;
 
+/** Represents a hermite spline of degree 5. */
 public class QuinticHermiteSpline extends Spline {
   private static SimpleMatrix hermiteBasis;
   private final SimpleMatrix m_coefficients;

wpimath/src/main/java/edu/wpi/first/math/spline/Spline.java

@@ -99,7 +99,10 @@ public abstract class Spline {
    * the value of x[2] is the second derivative in the x dimension.
    */
   public static class ControlVector {
+    /** The x components of the control vector. */
     public double[] x;
+
+    /** The y components of the control vector. */
     public double[] y;
 
     /**

wpimath/src/main/java/edu/wpi/first/math/spline/SplineHelper.java

@@ -10,6 +10,7 @@ import java.util.Arrays;
 import java.util.List;
 import org.ejml.simple.SimpleMatrix;
 
+/** Helper class that is used to generate cubic and quintic splines from user provided waypoints. */
 public final class SplineHelper {
   /** Private constructor because this is a utility class. */
   private SplineHelper() {}

wpimath/src/main/java/edu/wpi/first/math/spline/SplineParameterizer.java

@@ -56,6 +56,7 @@ public final class SplineParameterizer {
     }
   }
 
+  /** Exception for malformed splines. */
   public static class MalformedSplineException extends RuntimeException {
     /**
      * Create a new exception with the given message.