docs: add info about caching pages with different domains (#3793)

Author: cernymatej

docs/content/docs/02.guide/09.different-domains.md

@@ -169,4 +169,27 @@ The same requests when using the `'prefix_except_default'`{lang="ts-type"} strat
 - https://mydomain.com/pl -> https://mydomain.com/pl (pl language)
 - https://mydomain.com/ua -> https://mydomain.com/ua (ua language)
 - https://es.mydomain.com -> https://es.mydomain.com (es language)
-- https://fr.mydomain.com -> https://fr.mydomain.com (fr language)
+- https://fr.mydomain.com -> https://fr.mydomain.com (fr language)
+
+## Caching considerations with different domains
+
+When using different domains, make sure to configure caching properly so that
+responses are correctly separated per domain.  
+
+Because the same route may be served under multiple domains (e.g. `en.mydomain.com`
+and `fr.mydomain.com`), caches need to vary by the request host. Otherwise,
+a response generated for one domain could be reused on another, causing the wrong language to render,
+leading to hydration mismatches and visible flashes on the client.
+
+The recommended setup is to use `cache.varies: ['host']` in your route rules,
+so that the `host` header is included in the cache key:
+
+```diff [nuxt.config.ts]
+export default defineNuxtConfig({
+  routeRules: {
+-    '/': { swr: 60 },
++    '/': { swr: 60, cache: { varies: ['host'] } },
+  },
+  // ...
+})
+```