Update auth0-context.tsx

[frederikprijck]

Description

Clarify the fact that you need to set useRefreshTokensFallback to true when using refreshtokens in order for the iframe to be used.

Checklist

• [ ] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• [ ] All active GitHub checks for tests, formatting, and security are passing
• [ ] The correct base branch is being used, if not the default branch