[DO NOT MERGE][Doc] add explanations for setting bookie rack name

[Anonymitaet]

1. Add docs for Fix invalid rack name cause bookie join rack failed #13683

   This doc should be added to the code file and shown on the pulsar-admin website automatically.

   However, pulsar-admin bookies are not rendered successfully. I've reported the issue and @urfreespace is investigating it.

   Consequently:

   • I add the docs to the CLI tool page first. If pulsar-admin bookies can be shown on the pulsar-admin website, I'll add the explanations to the code file and delete the explanations on the CLI tool page.
   • Though there are many other commands and flags, I only add explanations for the set-bookie-rack command and rack flag since this kind of content should be generated automatically rather than updating manually. Adding them here is to show an important reminder to users.

2. Preview looks good:
   

[Anonymitaet]

@hangc0276 @Huanli-Meng @momo-jun could you please help review? Thanks

[hangc0276]

Good job.