Emphasize that only a single DeadLetter sequence is processed

Emphasize that only a single DeadLetter sequence is processed through
the process operations on the SequencedDeadLetterProcessor and
SequencedDeadLetterQueue.

#2021

messaging/src/main/java/org/axonframework/messaging/deadletter/SequencedDeadLetterProcessor.java

@@ -19,8 +19,8 @@ public interface SequencedDeadLetterProcessor<M extends Message<?>> {
     /**
      * Process a sequence of {@link DeadLetter dead-letters} matching the given {@code sequenceFilter}.
      * <p>
-     * Note that the {@code sequenceFilter} is <em>only</em> invoked for the first letter of a sequence, as the first
-     * entry blocks the entire sequence.
+     * Note that only a <em>single</em> matching sequence is processed! Furthermore, the {@code sequenceFilter} is
+     * <em>only</em> invoked for the first letter of a sequence, as the first entry blocks the entire sequence.
      *
      * @param sequenceFilter A filter for the first {@link DeadLetter dead-letter} entries of each sequence.
      * @return {@code true} if at least one {@link DeadLetter dead-letter} was processed successfully, {@code false}
@@ -30,6 +30,8 @@ public interface SequencedDeadLetterProcessor<M extends Message<?>> {
 
     /**
      * Process any sequence of {@link DeadLetter dead-letters} belonging to this component.
+     * <p>
+     * Note that only a <em>single</em> matching sequence is processed!
      *
      * @return {@code true} if at least one {@link DeadLetter dead-letter} was processed successfully, {@code false}
      * otherwise.

messaging/src/main/java/org/axonframework/messaging/deadletter/SequencedDeadLetterQueue.java

@@ -198,8 +198,8 @@ void requeue(@Nonnull DeadLetter<? extends M> letter,
      * the {@code sequenceFilter}. Will pick the oldest available sequence based on the {@link DeadLetter#lastTouched()}
      * field from every sequence's first entry.
      * <p>
-     * Note that only the first dead-letter is validated, because it is the blocker for the processing of the rest of
-     * the sequence.
+     * Note that only a <em>single</em> matching sequence is processed! Furthermore, only the first dead-letter is
+     * validated, because it is the blocker for the processing of the rest of the sequence.
      * <p>
      * Uses the {@link EnqueueDecision} returned by the {@code processingTask} to decide whether to
      * {@link #evict(DeadLetter)} or {@link #requeue(DeadLetter, UnaryOperator)} a dead-letter from the selected
@@ -227,6 +227,8 @@ boolean process(@Nonnull Predicate<DeadLetter<? extends M>> sequenceFilter,
      * the oldest available sequence based on the {@link DeadLetter#lastTouched()} field from every sequence's first
      * entry.
      * <p>
+     * Note that only a <em>single</em> matching sequence is processed!
+     * <p>
      * Uses the {@link EnqueueDecision} returned by the {@code processingTask} to decide whether to
      * {@link #evict(DeadLetter)} or {@link #requeue(DeadLetter, UnaryOperator)} the dead-letter. The
      * {@code processingTask} is invoked as long as letters are present in the selected sequence and the result of