docs: add watchEffect notes

[Alfred-Skyblue]

Description of Problem

I have observed that users often make some mistakes when using watchEffect, such as asynchronously accessing reactive variables, resulting in dependencies not being collected, or having an if statement with a false condition during the initial execution, leading to uncollected dependencies. These errors can cause watchEffect to malfunction. In this pull request, I have added some notes to help users avoid these mistakes.

relate: vuejs/core#9773 vuejs/core#9408 vuejs/core#2093

Proposed Solution

Additional Information

My English is not very good. You can correct my grammar or inaccurate wording at will. Thank you.

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[fabruex]

Hi, I saw this pull request and I would add this point from last comment by LinusBorg in vuejs/core#9773, I think it's very important and clear:

reactive dependencies are re-collected on each run of an effect. This is so that a) new dependencies, that might not have been accessed/read from in a previous run, can be read, and b) old dependencies, that are no longer being read from in the latest run, can be discarded.