GH-920: Suppress ERROR Log for Expected Exception

```
2021-02-25 15:44:31.640 ERROR 73473 --- [etry-8000-0-C-1] o.s.k.l.SeekToCurrentErrorHandler        : Failed to determine if this record (kgh920-retry-8000-0@5) should be recovererd, including in seeks
```

In `SeekUtils` when the nested exception is a `KafkaBackoffException`.

Also fix some asciidoctor rendering in the docs.

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/listener/SeekUtils.java

@@ -30,6 +30,7 @@
 import org.apache.kafka.common.TopicPartition;
 import org.apache.kafka.common.errors.SerializationException;
 
+import org.springframework.core.NestedRuntimeException;
 import org.springframework.core.log.LogAccessor;
 import org.springframework.kafka.KafkaException;
 import org.springframework.kafka.KafkaException.Level;
@@ -105,8 +106,15 @@ public static boolean doSeeks(List<ConsumerRecord<?, ?>> records, Consumer<?, ?>
 					skipped.set(test);
 				}
 				catch (Exception ex) {
-					logger.error(ex, "Failed to determine if this record (" + ListenerUtils.recordToString(record)
-							+ ") should be recovererd, including in seeks");
+					if (isBackoffException(ex)) {
+						logger.debug(ex, () -> ListenerUtils.recordToString(record)
+								+ " included in seeks due to retry back off");
+					}
+					else {
+						logger.error(ex, () -> "Failed to determine if this record ("
+								+ ListenerUtils.recordToString(record)
+								+ ") should be recovererd, including in seeks");
+					}
 					skipped.set(false);
 				}
 				if (skipped.get()) {
@@ -221,4 +229,15 @@ public static void seekOrRecover(Exception thrownException, List<ConsumerRecord<
 		}
 	}
 
+	/**
+	 * Return true if the exception is a {@link KafkaBackoffException}.
+	 * @param exception the exception.
+	 * @return true if it's a back off.
+	 * @since 2.7
+	 */
+	public static boolean isBackoffException(Exception exception) {
+		return NestedRuntimeException.class.isAssignableFrom(exception.getClass())
+				&& ((NestedRuntimeException) exception).contains(KafkaBackoffException.class);
+	}
+
 }

spring-kafka/src/main/java/org/springframework/kafka/retrytopic/DeadLetterPublishingRecovererFactory.java

@@ -29,7 +29,7 @@
 import org.springframework.core.NestedRuntimeException;
 import org.springframework.kafka.core.KafkaOperations;
 import org.springframework.kafka.listener.DeadLetterPublishingRecoverer;
-import org.springframework.kafka.listener.KafkaBackoffException;
+import org.springframework.kafka.listener.SeekUtils;
 import org.springframework.kafka.support.KafkaHeaders;
 
 /**
@@ -96,7 +96,7 @@ public void setDeadLetterPublishingRecovererCustomizer(Consumer<DeadLetterPublis
 	}
 
 	private TopicPartition resolveDestination(ConsumerRecord<?, ?> cr, Exception e) {
-		if (isBackoffException(e)) {
+		if (SeekUtils.isBackoffException(e)) {
 			throw (NestedRuntimeException) e; // Necessary to not commit the offset and seek to current again
 		}
 
@@ -112,11 +112,6 @@ private TopicPartition resolveDestination(ConsumerRecord<?, ?> cr, Exception e)
 				cr.partition() % nextDestination.getDestinationPartitions());
 	}
 
-	private boolean isBackoffException(Exception e) {
-		return NestedRuntimeException.class.isAssignableFrom(e.getClass())
-				&& ((NestedRuntimeException) e).contains(KafkaBackoffException.class);
-	}
-
 	private int getAttempts(ConsumerRecord<?, ?> consumerRecord) {
 		Header header = consumerRecord.headers().lastHeader(RetryTopicHeaders.DEFAULT_HEADER_ATTEMPTS);
 		return header != null