docs: clarify docs to address installation errors

[fivecar]

This addresses one category of issue that things like #12282 talk about. The original docs' instructions don't fix the uncleared-cache problem for people.

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[fivecar]

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[richardkmichael]

To try to install CRA v4.0.3, as in #12022 , I've read the diff here and tried the instructions, but as written I don't think they're helpful. I may have misunderstood them.

1. I uninstalled create-react-app. It does not appear in the local or global package list.
2. I ran npx clear-npx-cache.

$ npx clear-npx-cache
Need to install the following packages:
  clear-npx-cache@1.0.1
Ok to proceed? (y) y

Apparently installed a new package was installed, but nothing else seems to have happened. The clear-npx-cache package doesn't appear in the local or global package list, I'm unsure if this is expected.

4. I re-installed: npm install create-react-app@4.0.3. npm create-react-app app still refuses to run.