[Doc] add explanations for setting bookie rack name (#13913)

* [Doc] add explanations for setting bookie rack name

* update

* update

* update

* update

Author: Anonymitaet

pulsar-client-tools/src/main/java/org/apache/pulsar/admin/cli/CmdBookies.java

@@ -85,7 +85,13 @@ private class UpdateBookie extends CliCommand {
                 description = "Bookie address (format: `address:port`)", required = true)
         private String bookieAddress;
 
-        @Parameter(names = { "-r", "--rack" }, description = "Bookie rack name", required = true)
+        @Parameter(names = { "-r", "--rack" }, description = "Bookie rack name. "
+                + "If you set a bookie rack name to slash (/) "
+                + "or an empty string (\"\"): "
+                + "if you use Pulsar earlier than 2.7.5, 2.8.3, and 2.9.2, "
+                + "an an exception is thrown; "
+                + "if you use Pulsar later than 2.7.5, 2.8.3, and 2.9.2, "
+                + "it falls back to /default-rack or /default-region/default-rack.", required = true)
         private String bookieRack;
 
         @Parameter(names = { "--hostname" }, description = "Bookie host name", required = false)

site2/docs/administration-isolation.md

@@ -73,6 +73,10 @@ bin/pulsar-admin namespaces set-bookie-affinity-group public/default \
 --primary-group group-bookie1
 ```
 
+> **Note**
+> 
+> Do not set a bookie rack name to slash (`/`) or an empty string (`""`) if you use Pulsar earlier than 2.7.5, 2.8.3, and 2.9.2. For the bookie rack name restrictions, see [pulsar-admin bookies set-bookie-rack](https://pulsar.apache.org/tools/pulsar-admin/).
+
 <!--REST API-->
 
 [POST /admin/v2/namespaces/{tenant}/{namespace}/persistence/bookieAffinity](https://pulsar.apache.org/admin-rest-api/?version=master&apiversion=v2#operation/setBookieAffinityGroup)