[Improve] update Unsubscribe() interface comment

[geniusjoe]

Motivation

I recently notice that some users using Unsubscribe() interface to stop receiving messages from current subscribed topic. But this action can lead to losing messages if retention policy exceed.

Modifications

We can add some comment in the Unsubscribe interface, and this comment can base on native Java sdk client.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The schema: no
• The default values of configurations: no
• The wire protocol: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? N/A
• If a feature is not applicable for documentation, explain why? no
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation. N/A

[tisonkun]

Thank you!