chore: remove misleading docs for credentialsFromJSON JWT path

[rileykarson]

This appears to have drifted in #1096, when the check for zero scopes in isSelfSignedJWTFlow was removed. Per the current rules, you either need to provide the internal option https://pkg.go.dev/google.golang.org/api/option/internaloption#EnableJwtWithScope or an explicit audience

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[noahdietz]

Thanks @rileykarson

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