doc: Add notes to stress call-during-initialisation

[outloudvi]

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[outloudvi]

I'm sending this PR because I experienced such problems recently. I was using getContext() in onMount(), which is incorrect, but it was working in Svelte around 3.24.0. It doesn't work anymore in the new versions of Svelte (such as 3.29.4). Therefore, I think it would be helpful to stress this since getContext() is pretty popular.

[dummdidumm]

The paragraph was repetitive to a section above in the tutorial, so incorporated this stress-call up there.