[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:  request  |     Status:  new
 Priority:  Normal   |  Milestone:
Component:  website  |    Version:
 Keywords:           |       Port:
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 Neither https://guide.macports.org/#reference.keywords nor
 https://guide.macports.org/#reference.phases mention the keyword
 "known_fail".

 Please document the keyword so maintainers become aware of it and how to
 use it.

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