[docs] Add missing JavaDocs (#6146)

wpimath/src/main/java/edu/wpi/first/math/system/Discretization.java

@@ -9,6 +9,7 @@ import edu.wpi.first.math.Num;
 import edu.wpi.first.math.Pair;
 import org.ejml.simple.SimpleMatrix;
 
+/** Discretization helper functions. */
 public final class Discretization {
   private Discretization() {
     // Utility class

wpimath/src/main/java/edu/wpi/first/math/system/LinearSystem.java

@@ -8,6 +8,18 @@ import edu.wpi.first.math.Matrix;
 import edu.wpi.first.math.Num;
 import edu.wpi.first.math.numbers.N1;
 
+/**
+ * A plant defined using state-space notation.
+ *
+ * <p>A plant is a mathematical model of a system's dynamics.
+ *
+ * <p>For more on the underlying math, read
+ * https://file.tavsys.net/control/controls-engineering-in-frc.pdf.
+ *
+ * @param <States> Number of states.
+ * @param <Inputs> Number of inputs.
+ * @param <Outputs> Number of outputs.
+ */
 public class LinearSystem<States extends Num, Inputs extends Num, Outputs extends Num> {
   /** Continuous system matrix. */
   private final Matrix<States, States> m_A;

wpimath/src/main/java/edu/wpi/first/math/system/NumericalIntegration.java

@@ -11,6 +11,7 @@ import java.util.function.BiFunction;
 import java.util.function.DoubleFunction;
 import java.util.function.Function;
 
+/** Numerical integration utilities. */
 public final class NumericalIntegration {
   private NumericalIntegration() {
     // utility Class

wpimath/src/main/java/edu/wpi/first/math/system/NumericalJacobian.java

@@ -11,6 +11,7 @@ import edu.wpi.first.math.numbers.N1;
 import java.util.function.BiFunction;
 import java.util.function.Function;
 
+/** Numerical Jacobian utilities. */
 public final class NumericalJacobian {
   private NumericalJacobian() {
     // Utility Class.

wpimath/src/main/java/edu/wpi/first/math/system/plant/DCMotor.java

@@ -12,16 +12,34 @@ import edu.wpi.first.util.struct.StructSerializable;
 
 /** Holds the constants for a DC motor. */
 public class DCMotor implements ProtobufSerializable, StructSerializable {
+  /** Voltage at which the motor constants were measured. */
   public final double nominalVoltageVolts;
+
+  /** Torque when stalled. */
   public final double stallTorqueNewtonMeters;
+
+  /** Current draw when stalled. */
   public final double stallCurrentAmps;
+
+  /** Current draw under no load. */
   public final double freeCurrentAmps;
+
+  /** Angular velocity under no load. */
   public final double freeSpeedRadPerSec;
+
+  /** Motor internal resistance. */
   public final double rOhms;
+
+  /** Motor velocity constant. */
   public final double KvRadPerSecPerVolt;
+
+  /** Motor torque constant. */
   public final double KtNMPerAmp;
 
+  /** DCMotor protobuf for serialization. */
   public static final DCMotorProto proto = new DCMotorProto();
+
+  /** DCMotor struct for serialization. */
   public static final DCMotorStruct struct = new DCMotorStruct();
 
   /**

wpimath/src/main/java/edu/wpi/first/math/system/plant/LinearSystemId.java

@@ -12,6 +12,7 @@ import edu.wpi.first.math.numbers.N1;
 import edu.wpi.first.math.numbers.N2;
 import edu.wpi.first.math.system.LinearSystem;
 
+/** Linear system ID utility functions. */
 public final class LinearSystemId {
   private LinearSystemId() {
     // Utility class