docs: clarify HTTP header setting from layouts

[sharadvc]

Summary

The on-demand rendering caution said HTTP headers could only be set at the page level, not in layouts. That was too strict.

On on-demand routes, Astro.response.headers set from layouts and nested .astro components does reach the response if set before the body streams. Prerendered pages still ignore custom headers, and late/async updates after streaming starts may not apply.

Fixes #11843

[astrobot-houston]

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[ArmandPhilippot]

See #14541 (comment)