[docs] Add missing JavaDocs (#6146)

wpimath/src/main/java/edu/wpi/first/math/Pair.java

@@ -4,23 +4,54 @@
 
 package edu.wpi.first.math;
 
+/**
+ * Represents a pair of two objects.
+ *
+ * @param <A> The first object's type.
+ * @param <B> The second object's type.
+ */
 public class Pair<A, B> {
   private final A m_first;
   private final B m_second;
 
+  /**
+   * Constructs a pair.
+   *
+   * @param first The first object.
+   * @param second The second object.
+   */
   public Pair(A first, B second) {
     m_first = first;
     m_second = second;
   }
 
+  /**
+   * Returns the first object.
+   *
+   * @return The first object.
+   */
   public A getFirst() {
     return m_first;
   }
 
+  /**
+   * Returns the second object.
+   *
+   * @return The second object.
+   */
   public B getSecond() {
     return m_second;
   }
 
+  /**
+   * Returns a pair comprised of the two given objects.
+   *
+   * @param <A> The first object's type.
+   * @param <B> The second object's type.
+   * @param a The first object.
+   * @param b The second object.
+   * @return A pair comprised of the two given objects.
+   */
   public static <A, B> Pair<A, B> of(A a, B b) {
     return new Pair<>(a, b);
   }