[improve][doc]add more explanation for retention configuration (#16572)

Author: HQebupt

conf/broker.conf

@@ -1301,10 +1301,14 @@ replicatorPrefix=pulsar.repl
 # due to missing ZooKeeper watch (disable with value 0)
 replicationPolicyCheckDurationSeconds=600
 
-# Default message retention time
+# Default message retention time.
+# The default value is 0, which means the data is removed after all the subscriptions are consumed.
+# Value less than 0 means messages never expire.
 defaultRetentionTimeInMinutes=0
 
-# Default retention size
+# Default retention size.
+# The default value is 0, which means the data is removed after all the subscriptions are consumed.
+# Value less than 0 means no infinite size quota.
 defaultRetentionSizeInMB=0
 
 # How often to check whether the connections are still alive

pulsar-broker-common/src/main/java/org/apache/pulsar/broker/ServiceConfiguration.java

@@ -2378,12 +2378,14 @@ public class ServiceConfiguration implements PulsarConfiguration {
     private boolean replicationTlsEnabled = false;
     @FieldContext(
         category = CATEGORY_POLICIES,
-        doc = "Default message retention time"
+        doc = "Default message retention time. The default value is 0, which means the data is removed after all the "
+                + "subscriptions are consumed. Value less than 0 means messages never expire."
     )
     private int defaultRetentionTimeInMinutes = 0;
     @FieldContext(
         category = CATEGORY_POLICIES,
-        doc = "Default retention size"
+        doc = "Default retention size. The default value is 0, which means the data is removed after all the "
+                + "subscriptions are consumed. Value less than 0 means no infinite size quota."
     )
     private int defaultRetentionSizeInMB = 0;
     @FieldContext(