docs: 429 with exponential backoff fallback example

[iloveitaly]

This took me a bit to figure out. The wording on on_predicate made it sound like the predicate argument could be
either an exception or a value, but this is not the case.

[hhromic]

I was stuck for a while with this too, until I found this PR. Thanks!
For the record, I also got confused by the wording and also was struggling with on_predicate not processing exceptions.

The most important part for me from the content of this PR is to re-raise the exception inside the giveup handler to let it bubble-up to the next on_exception decorator and so on. Thanks for that tip!