docs: add note about runtime config serialization (#8432)

Co-authored-by: Daniel Roe <daniel@roe.dev>

Author: BracketJohn

docs/content/2.guide/4.going-further/10.runtime-config.md

@@ -39,6 +39,12 @@ console.log(runtimeConfig.public.apiBase)
 When using Options API the public runtime config is available via `this.$config.public`.
 ::
 
+### Serialization
+
+Your runtime config will be serialized before being passed to Nitro. This means that anything that cannot be serialized and then deserialized (such as functions, Sets, Maps, and so on), should not be set in your `nuxt.config`.
+
+Instead of passing non-serializable objects or functions into your application from your `nuxt.config`, you can place this code in a Nuxt or Nitro plugin or middleware.
+
 ### Environment Variables
 
 The most common way to provide configuration is by using [Environment Variables](https://medium.com/chingu/an-introduction-to-environment-variables-and-how-to-use-them-f602f66d15fa).