docs: Include warning when configuring token to install private packages

The distinction between `secrets.GITHUB_TOKEN` and `secrets.NPM_TOKEN` cost myself and a colleague numerous hours when we were trying to fix a GitHub Actions workflow which needed to install a private package from a different repository from our GitHub organisation.

Given the issue dedicated to this point is closed, we should include a warning here to make it more clear why `secrets.GITHUB_TOKEN` will not work when passed to `npm ci`, in the presence of private packages from other GitHub Package repositories.

Author: jamesottaway

Diff for: docs/advanced-usage.md

@@ -247,3 +247,5 @@ steps:
 # `npm rebuild` will run all those post-install scripts for us.
 - run: npm rebuild && npm run prepare --if-present
 ```
+
+NOTE: As per https://github.com/actions/setup-node/issues/49 you cannot use `secrets.GITHUB_TOKEN` to access private GitHub Packages within the same organisation.