Improve docs for token caching

[hadley]

How does it work, where do the token live, how can you customise, etc.

[hadley]

It looks like this is already documented at https://httr2.r-lib.org/articles/wrapping-apis.html#caching, but it took me a while to find it. I think it's worth making this a separate vignette, and then linking from the various oauth functions.