Clarify rewarded video availability check timing

[jk7287]

Changes

This surfaces the existing GetAdAvailabilityNowAsync timing guidance next to the client implementation steps. Developers should call the method as close as possible to showing the ad, such as when the user opens the shop menu containing the video ad button.

The best practices now also avoid calling GetAdAvailabilityNowAsync after an ad finishes unless another ad is being offered.

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[github-actions]

Hi @jk7287, thanks so much for helping improve the Roblox creator documentation! Our technical writing team will review your pull request soon. In the meantime, please ensure you've read through the README.md, contribution guidelines, and style recommendations.