docs: clarify which errors Astro error pages catch

[sharadvc]

Summary

The error prop on 500.astro is not “any error thrown during rendering.” Per maintainer guidance, it covers failures before the response body streams (route component script / middleware at await next()), not errors thrown while HTML is streamed from templates and components.

Update that wording and link to middleware.

Fixes #12172

[astrobot-houston]

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[ArmandPhilippot]

See #14541 (comment)