Document the validateWebhook interface on Node 18 #606

Job	Run time
build (20.x)	8s
test (node, 18.x)	22s
test (node, 20.x)	23s
test (node, 22.x)	20s
integration-bun (bun, 1.0.11)	6s
integration-browser (browser, chromium, 20.x)	39s
integration-deno (deno, v1.x)	10s
integration-edge (cloudflare-worker, 20.x)	18s
integration-browser (browser, firefox, 20.x)	39s
integration-nextjs (nextjs, 20.x)	28s
integration-browser (browser, webkit, 20.x)	57s
integration-node (commonjs, 18.x)	12s
integration-node (commonjs, 20.x)	10s
integration-node (esm, 18.x)	6s
integration-node (esm, 20.x)	16s
integration-node (typescript, 18.x)	11s
integration-node (typescript, 20.x)	9s
	5m 34s

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