[Doc][Improve] Backlog increase during subscription replication

[AlvaroStream]

Search before asking

• [X] I searched in the issues and found nothing similar.

What issue do you find in Pulsar docs?

When somebody uses replicated subscriptions sometimes can happen that there is a "ficticius" increase in backlog related to the storing snapshots overt the same topic if there are inactive subscriptions.

What is your suggestion?

We need to add that information as a Note on that section.

Any reference?

https://github.com/apache/pulsar/wiki/PIP-33:-Replicated-subscriptions#storing-snapshots

Are you willing to submit a PR?

• [X] I'm willing to submit a PR!