docs(optimize): mention session storage in auto preview note

[misiekhardcore]

related to OPT-7105

Description

When should this change go live?

• This change is not yet live and should not be merged until {ADD_DATE} (apply hold label or convert to draft PR)?
• There is no urgency with this change.
• This change or page is part of a marketing blog, conference talk, or something else on a schedule.
• This functionality is already available but undocumented.
• This is a bug fix or security concern.

PR Checklist

• I have added changes to the relevant /versioned_docs directory, or they are not for an already released version.
• I have added changes to the main /docs directory (aka /next/), or they are not for future versions.
• My changes require an Engineering review, and I've assigned an engineering manager or tech lead as a reviewer, or my changes do not require an Engineering review.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer, or my changes do not require a technical writer review.

[christinaausley]

Should this change also be applied to the current set and/or older versions of docs? Just want to confirm before approving.

[misiekhardcore]

@christinaausley nope, this is only for the next version. Thanks for keeping an eye on this tho!