[FW][FIX] subscriptions: modified detail re: prorated warning

[fw-bot]

Made requested adjustment from this PROJECT TASK: https://www.odoo.com/web#id=3859244&cids=3&model=project.task&view_type=form

• added requested important admonition to detail the prorated amount and how it operates in regards to different product types (lines 71-73)

Forward-Port-Of: odoo/documentation#9085

[robodoo]

Pull request status dashboard.

[fw-bot]

This PR targets saas-17.2 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port