Update rabbit_priority_queue.erl

[keyhanjk]

for better documenting according to this issue (not bug) erlang/otp#8685

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[lukebakken]

Please conform to the text width established throughout this file. Also, feel free to add erlang/otp#8685 in the comment itself to link to the issue.

[michaelklishin]

Thank you but is mentioned in the docs and I doubt most people would generally care beyond what's briefly covered there.