document SKIP_PREFLIGHT_CHECK, improve err messaging

Created by: eckdanny

Is your proposal related to a problem?

Unclear that PREFLIGHT_CHECK consists only of verifyPackageTree. CRA users in non-traditional environments (e.g.; git submodules, lerna, yarn workspaces) encountering this can get a false positive because normal node module resolution indeed works just fine.

refs:

• #1333
• #6756
• #1795 (closed)

Describe the solution you'd like

1. Wordsmith the error messaging in verifyPackageTree.js to indicate this is only a footgun check
2. Add something to the docs site about this flag. AFAIK, there is no mention of SKIP_PREFLIGHT_CHECK anywhere in the docs site, but adding to /docusaurus/docs/advanced-configuration.md at minimum would be a good idea.

Describe alternatives you've considered

Would be nice if the SKIP_PREFLIGHT_CHECK had a more descriptive name, but not worth changing IMO.