[MacPorts] #59640: Docs: keyword "known_fail" seems undocumented

[MacPorts]

#59640: Docs: keyword "known_fail" seems undocumented
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  Reporter:  lhaeger      |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  guide        |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign):

 We should still document that it exists, how we want it to be used, and
 what the consequences of using and not using it are.

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Ticket URL: <https://trac.macports.org/ticket/59640#comment:2>
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