Polish RabbitMQ properties doc

Fix description for `prefetch`; remove reference to transactions since
the direct container does not support transactionSize.

Also fix `transactionSize` to just reference acks; autoConfiguration
doesn't support enabling transactions.

Closes gh-14569

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/amqp/RabbitProperties.java

@@ -577,8 +577,8 @@ public class RabbitProperties {
 		private AcknowledgeMode acknowledgeMode;
 
 		/**
-		 * Number of messages to be handled in a single request. It should be greater than
-		 * or equal to the transaction size (if used).
+		 * Maximum number of unacknowledged messages that can be outstanding at each
+		 * consumer.
 		 */
 		private Integer prefetch;
 
@@ -659,9 +659,8 @@ public class RabbitProperties {
 		private Integer maxConcurrency;
 
 		/**
-		 * Number of messages to be processed in a transaction. That is, the number of
-		 * messages between acks. For best results, it should be less than or equal to the
-		 * prefetch count.
+		 * Number of messages to be processed between acks when the acknowledge mode is
+		 * AUTO. If larger than prefetch, prefetch will be increased to this value.
 		 */
 		private Integer transactionSize;