[docs] Add missing JavaDocs (#6125)

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

@@ -8,6 +8,7 @@ import edu.wpi.first.util.RuntimeLoader;
 import java.io.IOException;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+/** WPIMath JNI. */
 public final class WPIMathJNI {
   static boolean libraryLoaded = false;
   static RuntimeLoader<WPIMathJNI> loader = null;
@@ -385,15 +386,32 @@ public final class WPIMathJNI {
    */
   public static native String serializeTrajectory(double[] elements);
 
+  /** Sets whether JNI should be loaded in the static block. */
   public static class Helper {
     private static AtomicBoolean extractOnStaticLoad = new AtomicBoolean(true);
 
+    /**
+     * Returns true if the JNI should be loaded in the static block.
+     *
+     * @return True if the JNI should be loaded in the static block.
+     */
     public static boolean getExtractOnStaticLoad() {
       return extractOnStaticLoad.get();
     }
 
+    /**
+     * Sets whether the JNI should be loaded in the static block.
+     *
+     * @param load Whether the JNI should be loaded in the static block.
+     */
     public static void setExtractOnStaticLoad(boolean load) {
       extractOnStaticLoad.set(load);
     }
+
+    /** Utility class. */
+    private Helper() {}
   }
+
+  /** Utility class. */
+  private WPIMathJNI() {}
 }