Update javadocs for PscSink and PscSource to highlight difference com…

…pared to FlinkPscProducer and Consumer

psc-flink/src/main/java/com/pinterest/flink/connector/psc/sink/PscSink.java

@@ -36,6 +36,14 @@
 /**
  * Flink Sink to produce data into a PSC topicUri. The sink supports all delivery guarantees
  * described by {@link DeliveryGuarantee}.
+ *
+ * A PscSink using EXACTLY_ONCE delivery guarantee requires a {@link com.pinterest.flink.connector.psc.PscFlinkConfiguration#CLUSTER_URI_CONFIG}
+ * to be set in the producer config. This is used to pre-construct the {@link com.pinterest.psc.producer.PscBackendProducer} upon
+ * creation of a top-level {@link com.pinterest.psc.producer.PscProducer} to perform transactional operations. For this reason,
+ * a single PscSink instance cannot be used to write to topics spanning multiple different clusters. This limitation does not
+ * exist in the AT_LEAST_ONCE and NONE delivery guarantees, nor does it exist in {@link com.pinterest.flink.streaming.connectors.psc.FlinkPscProducer}
+ * which can be used to write to multiple clusters regardless of the delivery guarantee.
+ *
  * <li>{@link DeliveryGuarantee#NONE} does not provide any guarantees: messages may be lost in case
  *     of issues on the PubSub broker and messages may be duplicated in case of a Flink failure.
  * <li>{@link DeliveryGuarantee#AT_LEAST_ONCE} the sink will wait for all outstanding records in the

psc-flink/src/main/java/com/pinterest/flink/connector/psc/source/PscSource.java

@@ -62,7 +62,17 @@
 
 /**
  * The Source implementation of PSC. Please use a {@link PscSourceBuilder} to construct a {@link
- * PscSource}. The following example shows how to create a PscSource emitting records of <code>
+ * PscSource}
+ *
+ * This is different from {@link com.pinterest.flink.streaming.connectors.psc.FlinkPscConsumer}
+ * in that it is a source implementation for Flink's new source API. Due to the difference in implementation,
+ * the two classes are not compatible with each other. Most notably, PscSource currently can only support
+ * reading from topics from a single backend cluster, while FlinkPscConsumer can read from multiple clusters.
+ * This limitation is due to the fact that a {@link com.pinterest.flink.connector.psc.PscFlinkConfiguration#CLUSTER_URI_CONFIG}
+ * is required to be set in the configuration for the PscSource to perform metadata queries against the cluster using
+ * a {@link com.pinterest.psc.metadata.client.PscMetadataClient}.
+ *
+ * The following example shows how to create a PscSource emitting records of <code>
  * String</code> type.
  *
  * <pre>{@code