Clarified actor state persistence description

[WhitWaldo]

Description

The previous language used in the description in this quickstart was confusing and has spawned at least two questions about it that I've seen in the Discord channel. Updated to reflect that while changes to the state are made to the cache on a per-actor basis, but that an explicit call to SaveStateAsync is required to persist the state changes to the underlying actor state store.

I didn't make any code changes - just updated a comment in a method.

Issue reference

We strive to have all PRs being opened based on an issue, where the problem or feature have been discussed prior to implementation.

None: Question raised in Discord.

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[paulyuk]

This is a nice clarification. LGTM. Thank you for this @WhitWaldo