NIFI-4372 Wait processor - recommend prioritizer in documentation

[pvillard31]

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• Is there a JIRA ticket associated with this PR? Is it referenced
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Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

[markap14]

Thanks for pointing this out, @pvillard31. Confirmed that I was able to replicate the issue and that it is resolved once I use the FIFO Prioritizer. Have merged this changed to master.