chore: java doc update to for returned data instances (#165)

* chore: java doc update to for returned data instances

To let the user know about Batcher instance returned from Bulk operations are not threadsafe.

* chore: addressing feedback comments

Added JavaDocs for `BigtableDataClient`.

* Adding comments for BigtableDataClient#readRows()

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/BigtableDataClient.java

@@ -69,6 +69,9 @@
  * an application. However, close() needs to be called on the client object to clean up resources
  * such as threads during application shutdown.
  *
+ * <p>This client can be safely shared across multiple threads except for the Batcher instances
+ * returned from bulk operations, eg. `newBulkMutationBatcher()`, `newBulkReadRowsBatcher()`.
+ *
  * <p>The surface of this class includes several types of Java methods for each of the API's
  * methods:
  *
@@ -651,7 +654,8 @@ public <RowT> UnaryCallable<Query, RowT> readRowCallable(RowAdapter<RowT> rowAda
   }
 
   /**
-   * Convenience method for synchronously streaming the results of a {@link Query}.
+   * Convenience method for synchronously streaming the results of a {@link Query}. The returned
+   * ServerStream instance is not threadsafe, it can only be used from single thread.
    *
    * <p>Sample code:
    *
@@ -1045,7 +1049,8 @@ public void bulkMutateRows(BulkMutation mutation) {
 
   /**
    * Mutates multiple rows in a batch. Each individual row is mutated atomically as in MutateRow,
-   * but the entire batch is not executed atomically.
+   * but the entire batch is not executed atomically. The returned Batcher instance is not
+   * threadsafe, it can only be used from single thread.
    *
    * <p>Sample Code:
    *
@@ -1073,7 +1078,8 @@ public Batcher<RowMutationEntry, Void> newBulkMutationBatcher(@Nonnull String ta
 
   /**
    * Reads rows for given tableId in a batch. If the row does not exist, the value will be null.
-   * This operation should be called with in a single thread.
+   * This operation should be called with in a single thread. The returned Batcher instance is not
+   * threadsafe, it can only be used from single thread.
    *
    * <p>Sample Code:
    *
@@ -1107,7 +1113,8 @@ public Batcher<ByteString, Row> newBulkReadRowsBatcher(String tableId) {
 
   /**
    * Reads rows for given tableId and filter criteria in a batch. If the row does not exist, the
-   * value will be null. This operation should be called with in a single thread.
+   * value will be null. This operation should be called with in a single thread. The returned
+   * Batcher instance is not threadsafe, it can only be used from single thread.
    *
    * <p>Sample Code:
    *