docs(load): clarify universal and server load

[theetrain]

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• Provides details and examples on how universal and server load runs, in what order, and what data gets returned.

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[Rich-Harris]

I feel like this is already covered:

kit/documentation/docs/20-core-concepts/20-load.md

Line 179 in acec5e6

Universal `load` functions are called with a `LoadEvent`, which has a `data` property. If you have `load` functions in both `+page.js` and `+page.server.js` (or `+layout.js` and `+layout.server.js`), the return value of the server `load` function is the `data` property of the universal `load` function's argument.

kit/documentation/docs/20-core-concepts/20-load.md

Line 193 in acec5e6

In rare cases, you might need to use both together — for example, you might need to return an instance of a custom class that was initialised with data from your server.

I'm open to the idea that it could be improved but we should avoid being duplicative, or spending a significant amount of real estate on a somewhat rare use case

[theetrain]

It is covered, but comes with unanswered questions:

1. Are universal and server load methods called in parallel?
2. Which load method's return value will be passed to export let data?
3. Does the returned data between both loaders merge automatically the same way layout data does?
4. Is there a similar await server() method like await parent()?

All of which are answered in this PR.

I also considered opening a feature request for await server() (akin to await parent()) but I realize it's better to keep existing behaviour—with both load methods being called one after the other—that way the universal load can obtain all data from the server load. And if a user really wants to parallelize server and universal load methods, they can appoint all async calls to the server load.

[dummdidumm]

I'm ok with the text enhancements but I also don't know if the additional code snippet is worth it

[theetrain]

@dummdidumm I suppose this line adequately explains it:

You can combine server and universal load returned data by spreading the universal load's data property

Though since this concept isn't covered elsewhere (such as the tutorial), I feel the code snippet is warranted. I often feel generous when it comes to showing examples of concepts, and the Kit docs are comprehensive enough that we get to decide whether deeper explanations should belong in the tutorial, guide, or API sections.