GH-780: More 2.0.0 Doc Polishing

Author: garyrussell

Diff for: src/reference/asciidoc/appendix.adoc

@@ -49,6 +49,9 @@ When using `spring-kafka-test` (version 2.1.x, starting with version 2.1.5) with
 
 Starting with version 2.1.9, the 2.0.0 `kafka-clients` jar can be used with similar overrides to those described above.
 
+IMPORTANT: When using the 2.0.0 clients with `spring-kafka` 2.1.x, the framework will continue to use the blocking `poll(long)` method to fetch records (future releases will use the `poll(Duration)` method).
+Other previously blocking consumer APIs will timeout according to the `ConsumerConfig.DEFAULT_API_TIMEOUT_MS_CONFIG` (`default.api.timeout.ms`) property; refer to the https://kafka.apache.org/documentation/#upgrade_200_notable[Notable Changes in 2.0.0] section in the Kafka documentation for more information.
+
 [appendix]
 [[history]]
 == Change History

Diff for: src/reference/asciidoc/quick-tour.adoc

@@ -34,6 +34,9 @@ compile 'org.springframework.kafka:spring-kafka:{spring-kafka-version}'
 
 NOTE: The framework has been tested with `kafka-clients` 1.1.1 and 2.0.0; when using the `spring-kafka-test` embedded Kafka broker, you must override other jars, as described in <<deps-for-11x>>.
 
+IMPORTANT: When using the 2.0.0 clients with `spring-kafka` 2.1.x, the framework will continue to use the blocking `poll(long)` method to fetch records (future releases will use the `poll(Duration)` method).
+Other previously blocking consumer APIs will timeout according to the `ConsumerConfig.DEFAULT_API_TIMEOUT_MS_CONFIG` (`default.api.timeout.ms`) property; refer to the https://kafka.apache.org/documentation/#upgrade_200_notable[Notable Changes in 2.0.0] section in the Kafka documentation for more information.
+
 ===== Very, Very Quick
 
 Using plain Java to send and receive a message: