[8.x] Document new ThrottlesExceptions job middleware

[paras-malhotra]

Documentation for laravel/framework#36473 and laravel/framework#36518 for the upcoming release.

If laravel/framework#36518 isn't merged before the upcoming release, we can just delete the last "tip" line from this PR.

[taylorotwell]

@paras-malhotra I've made some changes to this feature since original merge. First, the job's uniquely assigned UUID is the default cache key and may be overwritten by the by method. Second, the "retry after minutes" has been changed to backoff and is specified in seconds instead of minutes to allow for a more granular approach.

[paras-malhotra]

@taylorotwell wouldn't setting the cache key as the job UUID mean that subsequent jobs of the same class would not be short circuited? So, say I have a job that hits an API for each user. The API is down and I need all subsequent jobs (of the same class but maybe for different users) to be short circuited to give enough time for the service to recover.

I think it might be better to have this set to the job class because I think it may serve as a better default for this middleware.

[taylorotwell]

@paras-malhotra yeah I could see your reasoning there - let me take a look at it again.

[taylorotwell]

I've updated it to use the class name by default and added a byJob method to quickly indicate it should use the UUID:

new ThrottlesException(...)->byJob()