Properly document jgvariant-ostree.

Change-Id: I0aa3b1df512ef99d0e25d73efdd34a1b488e7d0d
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DeltaFallback.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DeltaFallback.java
index 1b3cc0a..46470d4 100644
--- a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DeltaFallback.java
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/DeltaFallback.java
@@ -6,19 +6,31 @@
 /**
  * A fallback entry in a {@link DeltaSuperblock}.
  *
+ * <p>References a file in the OSTree repository.
+ *
  * <p>Reference: {@code ostree-repo-static-delta-private.h#OSTREE_STATIC_DELTA_FALLBACK_FORMAT}
+ *
+ * @param objectType the object type of the file represented by this fallback entry.
+ * @param checksum the checksum of the file represented by this fallback entry.
+ * @param compressedSize the compressed size of the file represented by this fallback entry.
+ * @param uncompressedSize the uncompressed size of the file represented by this fallback entry.
  */
 public record DeltaFallback(
-    byte objectType, Checksum checksum, long compressedSize, long uncompressedSize) {
+    ObjectType objectType, Checksum checksum, long compressedSize, long uncompressedSize) {
 
   private static final Decoder<DeltaFallback> DECODER =
       Decoder.ofStructure(
           DeltaFallback.class,
-          Decoder.ofByte(),
+          Decoder.ofByte().map(ObjectType::valueOf),
           Checksum.decoder(),
           Decoder.ofLong().withByteOrder(ByteOrder.LITTLE_ENDIAN), // FIXME: non-canonical
           Decoder.ofLong().withByteOrder(ByteOrder.LITTLE_ENDIAN)); // FIXME: non-canonical
 
+  /**
+   * Acquires a {@link Decoder} for the enclosing type.
+   *
+   * @return a possibly shared {@link Decoder}.
+   */
   public static Decoder<DeltaFallback> decoder() {
     return DECODER;
   }