Update Update-AzReservation.md (#23269)

* Update Update-AzReservation.md

This should be described since it's hard to figure out these.

* Update Update-AzReservation.md

---------

Co-authored-by: Yabo Hu <[email protected]>

Author: yikeura700

src/Reservations/help/Update-AzReservation.md

@@ -317,7 +317,7 @@ Accept wildcard characters: False
 ```
 
 ### -Renew
-Setting this to true will automatically purchase a new reservation on the expiration date time.
+Setting this to true will automatically purchase a new reservation on the expiration date time. Can be selected by -Renew:$true or -Renew:$false.
 
 ```yaml
 Type: System.Management.Automation.SwitchParameter