Improve documentation on Stores internals

[fredguth]

Describe the problem

I'm frustrated that it is not clear for a newbie what a start callback is. It is clear that a Readable that doesn't change is not useful, but there is no metaphor in the documentation explaining start. There are good examples, though.

Besides, the source code doesn't help either. For example:

svelte/src/runtime/store/index.ts

Line 12 in 5a3a1e4

/** Cleanup logic callback. */

/** Cleanup logic callback. */
type Invalidator<T> = (value?: T) => void;

What information does this description give?

After a long time trying to understand what an Invalidator is, I think it is about making stores compatible with rxjs, but this is not clear from the documentation nor the source code.

Describe the proposed solution

I would like to see a better metaphor about the start callback in the documentation. And references in the source-code to what Invalidator, StartStopNotifier are. The current definitions in the source code are "tautologic", for example:
A StartStopNotifier Start and stop notification callbacks..

Alternatives considered

A blog post.

Importance

nice to have

[samal-rasmussen]

More info on invalidate on StackOverflow:
https://stackoverflow.com/questions/76915395/what-is-the-second-invalidate-argument-to-readable-subscribe

Seems like an implementation detail to fix glitchy derived stores. Is there any use case for the public? If not then it should be removed from the docs, right?