[docs] Add missing JavaDocs (#6146)

wpiutil/src/main/java/edu/wpi/first/util/sendable/SendableBuilder.java

@@ -15,10 +15,14 @@ import java.util.function.LongConsumer;
 import java.util.function.LongSupplier;
 import java.util.function.Supplier;
 
+/** Helper class for building Sendable dashboard representations. */
 public interface SendableBuilder extends AutoCloseable {
   /** The backend kinds used for the sendable builder. */
   enum BackendKind {
+    /** Unknown. */
     kUnknown,
+
+    /** NetworkTables. */
     kNetworkTables
   }
 

wpiutil/src/main/java/edu/wpi/first/util/sendable/SendableRegistry.java

@@ -506,6 +506,9 @@ public final class SendableRegistry {
 
     /** Sendable builder for the sendable. */
     public SendableBuilder builder;
+
+    /** Default constructor. */
+    public CallbackData() {}
   }
 
   // As foreachLiveWindow is single threaded, cache the components it