[improve][doc] Updated set retention time documentation (#18654)

site2/docs/reference-pulsar-admin.md

@@ -1409,7 +1409,7 @@ Options
 |Flag|Description|Default|
 |----|---|---|
 |`-s`, `--size`|The retention size limits (for example 10M, 16G or 3T) for each topic in the namespace. 0 means no retention and -1 means infinite size retention||
-|`-t`, `--time`|The retention time in minutes, hours, days, or weeks. Examples: 100m, 13h, 2d, 5w. 0 means no retention and -1 means infinite time retention||
+|`-t`, `--time`|The retention time with optional time unit suffix. For example, 100m, 3h, 2d, 5w. If the time unit is not specified, the default unit is seconds. For example, -t 120 sets retention to 2 minutes. 0 means no retention and -1 means infinite time retention||
 
 
 ### `unload`