Document what higher/lower priority numbers mean in ActiveJob guide

Or rather, what they could mean. What to do with priority numbers is up to the adapter implementation.

All adapters that support priority that I know of treat lower priority as more urgent:
* Backburner
* Delayed Job
* Que
* SolidQueue
* GoodJob (in v4)

Author: Earlopain

guides/source/active_job_basics.md

@@ -357,6 +357,10 @@ You can also pass a `:priority` option to `set`:
 MyJob.set(priority: 50).perform_later(record)
 ```
 
+NOTE: If a lower priority number performs before or after a higher priority number depends on the
+adapter implementation. Refer to documentation of your backend for more information.
+Adapter authors are encouraged to treat a lower number as more important.
+
 [`queue_with_priority`]: https://api.rubyonrails.org/classes/ActiveJob/QueuePriority/ClassMethods.html#method-i-queue_with_priority
 
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