[Doc] Fix outdated isLoading in Data Provider chapter

[smeng9]

The doc states:

As a consequence, you should always use isPending to determine if you need to show a loading indicator.

It should be consistent with itself.

Probably there are some additional things needs to be discussed and fixed

1. Some of the stories still uses isLoading
2. useList still uses isLoading here

   react-admin/packages/ra-core/src/controller/list/useList.ts

   Line 173 in f2fc70f

   if (isLoading || !data) return;

3. In some enterprise components we pass isPending to the isLoading prop which is confusing

   react-admin/docs/Features.md

   Line 988 in f2fc70f

   return <Timeline isLoading={isPending} records={data} />;

Additional Checks

• The PR targets master for a bugfix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
• The documentation is up to date

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[djhi]

Thanks!

[fzaninotto]

Thanks for your PR! And you're right, we have some work left on the documentation.

In useList, checking isLoading is a mistake. Would you like to open a PR to fix it?

Enterprise Edition components have recently been updated to react-admin v5 and should use isPending. The doc about EE components is probably not up to date (it is though at https://react-admin-ee.marmelab.com/whats-new). We'll do a pass to fix the remaining isLoading, but you're welcome to give us a hand!