Add @since tags.

gson/src/main/java/com/google/gson/Gson.java

@@ -366,6 +366,7 @@ public final class Gson {
    *
    * @throws IllegalArgumentException if this GSON cannot serialize and
    *     deserialize {@code type}.
+   * @since 2.1
    */
   public <T> TypeAdapter<T> getNextAdapter(TypeAdapter.Factory skipPast, TypeToken<T> type) {
     boolean skipPastFound = false;

gson/src/main/java/com/google/gson/GsonBuilder.java

@@ -468,6 +468,8 @@ public final class GsonBuilder {
    * adapter needs to be configured based on the type of the field being processed. Gson
    * is designed to handle a large number of factories, so you should consider registering
    * them to be at par with registering an individual type adapter.
+   *
+   * @since 2.1
    */
   public GsonBuilder registerTypeAdapterFactory(TypeAdapter.Factory factory) {
     factories.add(factory);

gson/src/main/java/com/google/gson/TypeAdapter.java

@@ -29,6 +29,10 @@ import java.io.Writer;
 
 // TODO: nice documentation
 
+/**
+ *
+ * @since 2.1
+ */
 public abstract class TypeAdapter<T> {
   public abstract T read(JsonReader reader) throws IOException;
   public abstract void write(JsonWriter writer, T value) throws IOException;
@@ -75,6 +79,9 @@ public abstract class TypeAdapter<T> {
     }
   }
 
+  /**
+   * @since 2.1
+   */
   public interface Factory {
     <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type);
   }