[doc] BookKeeper metadata service url should be documented when init metadata

[BewareMyPower]

#7196 introduced a command argument bookkeeper-metadata-service-uri to specify an existed BookKeeper metadata service uri so that a new broker cluster could reuse the existed ZooKeeper node to store bookies' metadata.

It should be documented like other arguments, e.g. see the table in https://pulsar.apache.org/docs/en/deploy-bare-metal/#initialize-cluster-metadata.

Also, the argument may be mistaken as a path to create a new ZooKeeper node to store bookies' metadata. So the doc should emphasize it or just change its name because the argument is not in any current release.

[murong00]

@BewareMyPower Thanks for pointing this out, I will refine this later when available.