Add background job syncing instructions

[loqimean]

Added instructions for background job syncing to ensure proper Webhook functionality.

Pull Request

Summary:
Added a short reminder/info about required background jobs for this code in docs.

Related Issue:
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Description:
I was struggled why I have no subscriptions and charges on my user, discovered every documentation and video tutor, but didn't find any answer, then I found it out that there are sync jobs, that is related to jobs.. and of course for this you need a running cli worker. When I started a worker my user received everything I need - subscriptions, charges.

Testing:
n/a

Screenshots (if applicable):
n/a

Checklist:

• Code follows the project's coding standards
• Tests have been added or updated to cover the changes
• Documentation has been updated (if applicable)
• All existing tests pass
• Conforms to the contributing guidelines

Additional Notes:
Might be it's not so clear, if it is, I am open for proposals. I hope this info will be helpful for community and prevent the same issue I had.