[Doc] Update the tenant permission modifications for pulsar-admin

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### What issue do you find in Pulsar docs?

The pulsar-admin tenants update command allows you to make someone a tenant admin, but the doc https://pulsar.apache.org/docs/next/admin-api-permissions/ does not show this.

### What is your suggestion?

Update the docs to add in the tenant permission modifications.

### Any reference?

//cc @illegalnumbers @vikram-narayan

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

Open the admin-api-permissions documentation page linked in the issue and review the pulsar-admin tenants update command behavior. Add the missing tenant permission modification details, then verify that the documentation accurately describes how the command can make someone a tenant admin.

Written by the indexing model from the issue text.