NIFI-5875 Improve docs around the PriorityAttributePrioritizer

[emilyselwood]

Clear up the documentation around the PriorityAttributePrioritizer to make it clearer that if the attributes are both number the largest wins but if they are not numbers they sort the other way around.

I had to go and look at the code to work out what was going on after reading the existing documentation.

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[ijokarumawak]

@wselwood Thanks for clarifying docs! Although I agree with the idea of adding more details, the proposed updated doc could be too specific for some users.

I'd suggest leaving the original sentence as it is, and add some notes for detailed behavior. Thoughts?

[ijokarumawak]

If the attributes can be parsed as a long, they will be ordered largest number first. For example 9 will come before 1 and 100 will be selected before 99

Is this true? As following test case shows, if 1 and 2 are compared, 1 goes first.
https://github.com/apache/nifi/blob/master/nifi-nar-bundles/nifi-framework-bundle/nifi-framework/nifi-standard-prioritizers/src/test/java/org/apache/nifi/prioritizer/PriorityAttributePrioritizerTest.java#L91

[emilyselwood]

Ouch. Good spot. The old docs got me spun around there. I'll update.

[ijokarumawak]

@wselwood Thanks for the update, looks much better! For the even better readability, can we improve structure a little bit more organized? How about something like this?

Example output:

---

(As plain text)
PriorityAttributePrioritizer: Given two FlowFiles that both have a "priority" attribute, the one that has the highest priority value will be processed first.

• Note that an UpdateAttribute processor should be used to add the "priority" attribute to the FlowFiles before they reach a connection that has this prioritizer set.
• If only one has that attribute it will go first.
• Values for the "priority" attribute may be alphanumeric, where "a" is a higher priority than "z", and "1" is a higher priority than "9", for example.
• If "priority" attribute cannot be parsed as a long, unicode string ordering will be used. For example: "99" and "100" will be ordered so the flowfile with "99" comes first, but "A-99" and "A-100" will sort so the flowfile with "A-100" comes first.

[ijokarumawak]

Looks perfect, +1 merging. Thanks @wselwood !