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

[AlvaroStream]

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• [X] I searched in the 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

Are you willing to submit a PR?

• [X] I'm willing to submit a PR!