NIFI-13736 update Expression Language Guide, join function

[markobean]

Summary

Minor fix to the expected results of an Expression Language use of the join function. Update to Expression Language Guide. Documentation update only.

NIFI-13736

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  • JDK 21

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[markobean]

I added one more commit for additional consistencies. I made "good bye" correctly a single word, "goodbye". Also, removed reference to the attribute "filename" which is not used or necessary in the given examples.

[exceptionfactory]

Thanks for the documentation correction and improvement @markobean! +1 merging