docs: add tip when mixing universal and server load

[theetrain]

Relates to #11458

• Since modifying data to include non-serializable POJOs (such as components) will throw an error, this change provides a hint for users not to do that.

I came across an issue on Discord where someone tried using dynamic import() to load a component and pass it to data in the universal load:

export async function load({ data }) {
  data.component = (await import('$lib/components/Button.svelte')).default
  return data
  //     ^ error
}

This threw an error along the lines of "cannot return non-serializable POJOs" since data was being returned by the server load in +page.server.js. Hopefully my contribution to the docs helps point out this lesser-known nuance.

I see this as a nice-to-have, so I'll understand if you don't wish to merge it.

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[Conduitry]

@eltigerchino solicited opinions about this in the maintainers' chat, and my opinion (which I see @PatrickG also commented with above) was that we should instead serialize the server load function data with devalue before sending it on to the universal load function, so it's not tainted with any of the subsequent mutations from the universal load.

[benmccann]

I just noticed https://github.com/sveltejs/kit/pull/12235, which if merged would close this PR, so that one should probably be reviewed first

[teemingc]

I just noticed #12235, which if merged would close this PR, so that one should probably be reviewed first

Unfortunately, I couldn’t figure out how to serialise the server load data before running the universal load functions without some major refactoring. A simple fix would be to clone the server data object before passing it to the universal load but that’s not ideal.