Adding documentation for WriteBatch size limit (#392)

Author: Brian Chen

firebase-firestore/src/main/java/com/google/firebase/firestore/FirebaseFirestore.java

@@ -263,6 +263,10 @@ public Query collectionGroup(@NonNull String collectionId) {
    * transaction. If any document read within the transaction has changed, the updateFunction will
    * be retried. If it fails to commit after 5 attempts, the transaction will fail.
    *
+   * <p>The maximum number of writes allowed in a single transaction batch is 500, but note that
+   * each usage of FieldValue.serverTimestamp(), FieldValue.arrayUnion(), FieldValue.arrayRemove(),
+   * or FieldValue.increment() inside a transaction counts as an additional write.
+   *
    * @param updateFunction The function to execute within the transaction context.
    * @param executor The executor to run the transaction callback on.
    * @return The task returned from the updateFunction.
@@ -306,6 +310,10 @@ public <TResult> Task<TResult> runTransaction(
   /**
    * Creates a write batch, used for performing multiple writes as a single atomic operation.
    *
+   * <p>The maximum number of writes allowed in a single transaction batch is 500, but note that
+   * each usage of FieldValue.serverTimestamp(), FieldValue.arrayUnion(), FieldValue.arrayRemove(),
+   * or FieldValue.increment() inside a transaction counts as an additional write.
+   *
    * @return The created WriteBatch object.
    */
   @NonNull