Enhance Stripe billing portal documentation adding return_url option

[Mth0158]

Pull Request

Summary:
I wanted to customize the return url used by Stripe checkout UI:

Related Issue:
N/A

Description:
The gem allows it, but it's not documented, hence my PR.

pay/app/models/pay/stripe/customer.rb

Line 226 in 33b8c2e

def billing_portal(**options)

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:
N/A