Fix description of checkpoint-interval

See gh-12181

spring-boot-project/spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -780,7 +780,7 @@ content into your application. Rather, pick only the properties that you need.
 	spring.jta.atomikos.datasource.xa-data-source-class-name= # Vendor-specific implementation of XAConnectionFactory.
 	spring.jta.atomikos.datasource.xa-properties= # Vendor-specific XA properties.
 	spring.jta.atomikos.properties.allow-sub-transactions=true # Specify whether sub-transactions are allowed.
-	spring.jta.atomikos.properties.checkpoint-interval=500 # Interval between checkpoints, in milliseconds.
+	spring.jta.atomikos.properties.checkpoint-interval=500 # Interval between checkpoints, expressed as the number of log writes between two checkpoint.
 	spring.jta.atomikos.properties.default-jta-timeout=10000ms # Default timeout for JTA transactions.
 	spring.jta.atomikos.properties.default-max-wait-time-on-shutdown=9223372036854775807 # How long should normal shutdown (no-force) wait for transactions to complete.
 	spring.jta.atomikos.properties.enable-logging=true # Whether to enable disk logging.

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/jta/atomikos/AtomikosProperties.java

@@ -99,8 +99,9 @@ public class AtomikosProperties {
 	private String logBaseDir;
 
 	/**
-	 * Interval between checkpoints. A checkpoint reduces the log file size at the expense
-	 * of adding some overhead in the runtime.
+	 * Interval between checkpoints, expressed as the number of log writes between two
+	 * checkpoint. A checkpoint reduces the log file size at the expense of adding some
+	 * overhead in the runtime.
 	 */
 	private long checkpointInterval = 500;