Mention risks related to downsizing disk on backup pages

[AnouckColson]

Where on docs.platform.sh should be changed?

https://docs.platform.sh/environments/backup.html https://docs.platform.sh/environments/restore.html

What exactly should be updated?

The warning issued here => https://docs.platform.sh/create-apps/app-reference/single-runtime-image.html#downsize-a-disk should be mentioned on both pages dedicated to backups to make sure the information is highly visible.

Also we can include that any existing old backups from before any disk changes can still be used if one reverts the disk changes.

Additional context

No response