[docs] Add missing JavaDocs (#6146)

wpimath/src/main/java/edu/wpi/first/math/controller/ArmFeedforward.java

@@ -14,12 +14,22 @@ import edu.wpi.first.util.struct.StructSerializable;
  * against the force of gravity on a beam suspended at an angle).
  */
 public class ArmFeedforward implements ProtobufSerializable, StructSerializable {
+  /** The static gain, in volts. */
   public final double ks;
+
+  /** The gravity gain, in volts. */
   public final double kg;
+
+  /** The velocity gain, in volt seconds per radian. */
   public final double kv;
+
+  /** The acceleration gain, in volt seconds² per radian. */
   public final double ka;
 
+  /** Arm feedforward protobuf for serialization. */
   public static final ArmFeedforwardProto proto = new ArmFeedforwardProto();
+
+  /** Arm feedforward struct for serialization. */
   public static final ArmFeedforwardStruct struct = new ArmFeedforwardStruct();
 
   /**

wpimath/src/main/java/edu/wpi/first/math/controller/DifferentialDriveWheelVoltages.java

@@ -11,16 +11,29 @@ import edu.wpi.first.util.struct.StructSerializable;
 
 /** Motor voltages for a differential drive. */
 public class DifferentialDriveWheelVoltages implements ProtobufSerializable, StructSerializable {
+  /** Left wheel voltage. */
   public double left;
+
+  /** Right wheel voltage. */
   public double right;
 
+  /** DifferentialDriveWheelVoltages protobuf for serialization. */
   public static final DifferentialDriveWheelVoltagesProto proto =
       new DifferentialDriveWheelVoltagesProto();
+
+  /** DifferentialDriveWheelVoltages struct for serialization. */
   public static final DifferentialDriveWheelVoltagesStruct struct =
       new DifferentialDriveWheelVoltagesStruct();
 
+  /** Default constructor. */
   public DifferentialDriveWheelVoltages() {}
 
+  /**
+   * Constructs a DifferentialDriveWheelVoltages.
+   *
+   * @param left Left wheel voltage.
+   * @param right Right wheel voltage.
+   */
   public DifferentialDriveWheelVoltages(double left, double right) {
     this.left = left;
     this.right = right;

wpimath/src/main/java/edu/wpi/first/math/controller/ElevatorFeedforward.java

@@ -17,12 +17,22 @@ import edu.wpi.first.util.struct.StructSerializable;
  * against the force of gravity).
  */
 public class ElevatorFeedforward implements ProtobufSerializable, StructSerializable {
+  /** The static gain. */
   public final double ks;
+
+  /** The gravity gain. */
   public final double kg;
+
+  /** The velocity gain. */
   public final double kv;
+
+  /** The acceleration gain. */
   public final double ka;
 
+  /** ElevatorFeedforward protobuf for serialization. */
   public static final ElevatorFeedforwardProto proto = new ElevatorFeedforwardProto();
+
+  /** ElevatorFeedforward struct for serialization. */
   public static final ElevatorFeedforwardStruct struct = new ElevatorFeedforwardStruct();
 
   /**

wpimath/src/main/java/edu/wpi/first/math/controller/SimpleMotorFeedforward.java

@@ -10,8 +10,13 @@ import edu.wpi.first.math.system.plant.LinearSystemId;
 
 /** A helper class that computes feedforward outputs for a simple permanent-magnet DC motor. */
 public class SimpleMotorFeedforward {
+  /** The static gain. */
   public final double ks;
+
+  /** The velocity gain. */
   public final double kv;
+
+  /** The acceleration gain. */
   public final double ka;
 
   /**