docs: ErrorBoundary makes erroring routes uncacheable — document after RC

An SSR-caught <ErrorBoundary> error still responds 200, so the router sends Cache-Control: no-store and skips its SSR cache (#8745). A route is not cached while it errors, which can increase SSR costs if the error happens to all active users all at once.

Document after RC: the cost, and the way out — throw the failure as an error response (httpError()) instead of an inline fallback.