[Doc] PIP-356: Improve "Support Geo-Replication starts at earliest position" doc

### 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?

Geo-replication of a topic with `earliest` needs an unload not documented as seen in https://github.com/apache/pulsar/blob/master/pip/pip-356.md#motivation

### What is your suggestion?

Add this to the steps on the note

### Any reference?

The [PIP](https://github.com/apache/pulsar/blob/master/pip/pip-356.md)

### Are you willing to submit a PR?

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

Read the motivation section of pip/pip-356.md and use its geo-replication guidance to locate the related documentation note. Add the missing unload step to that note's procedure, then verify that the documented steps cover topics using earliest position.

Written by the indexing model from the issue text.