Mention producers are thread-safe

Author: acogoluegnes

src/docs/asciidoc/api.adoc

@@ -328,6 +328,13 @@ include::{test-examples}/ProducerUsage.java[tag=producer-creation]
 Consider a `Producer` instance like a long-lived object, do not create one
 to send just one message.
 
+
+[NOTE]
+.Producer thread safety
+====
+Producer instances are thread-safe.
+====
+
 Internally, the `Environment` will query the broker to find out about
 the topology of the stream and will create or re-use a connection to
 publish to the leader node of the stream.

src/main/java/com/rabbitmq/stream/Producer.java

@@ -18,6 +18,8 @@
  *
  * <p>Instances are created and configured with a {@link ProducerBuilder}.
  *
+ * Implementations are expected to be thread-safe, RabbitMQ Streams' are.
+ *
  * @see ProducerBuilder
  * @see Environment#producerBuilder()
  */