Document thread safety

moshi/src/main/java/com/squareup/moshi/JsonAdapter.java

@@ -30,6 +30,11 @@ import okio.BufferedSource;
 
 /**
  * Converts Java values to JSON, and JSON values to Java.
+ *
+ * <p>JsonAdapter instances provided by Moshi are thread-safe, meaning multiple threads can safely
+ * use a single instance concurrently.
+ *
+ * <p>Custom JsonAdapter implementations should be designed to be thread-safe.
  */
 public abstract class JsonAdapter<T> {
   @CheckReturnValue public abstract @Nullable T fromJson(JsonReader reader) throws IOException;

moshi/src/main/java/com/squareup/moshi/Moshi.java

@@ -39,6 +39,9 @@ import static com.squareup.moshi.internal.Util.typeAnnotatedWithAnnotations;
 
 /**
  * Coordinates binding between JSON values and Java objects.
+ *
+ * <p>Moshi instances are thread-safe, meaning multiple threads can safely use a single instance
+ * concurrently.
  */
 public final class Moshi {
   static final List<JsonAdapter.Factory> BUILT_IN_FACTORIES = new ArrayList<>(5);