clarify documentation comment for getTokenSilently

[jdugan1024]

Changes

This clarifies the documentation for getTokenSilently. I had to dig through the code a bit to find out
if would try to get a new token before the old one expired. Based on my reading of the code I updated
the docs to explain the behavior more clearly.

References

There is a comment in the code stating that it will try to get a new token 60 seconds before expiration: https://github.com/auth0/auth0-spa-js/blob/master/src/Auth0Client.ts#L1245

Testing

This is a documentation only change, so no tests were added or updated.

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All code quality tools/guidelines have been run/followed

[stevehobbsdev]

Thanks for your contribution here, just left a couple of small tweaks and suggestions.

[jdugan1024]

@stevehobbsdev have committed all of your suggestions. Thanks for the quick turnaround on this!

[stevehobbsdev]

Thanks!