[MacPorts] #39939: Document the "subport=" command line option (was: Documentation: subports)

[MacPorts]

#39939: Document the "subport=" command line option
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  Reporter:  software@…   |      Owner:  markd@…
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  guide        |    Version:  2.2.0
Resolution:               |   Keywords:
      Port:               |
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Comment (by larryv@…):

 Replying to [comment:3 egall@…]:
 > This is a duplicate of #36957

 This isn’t really the same. The other ticket is about documenting the use
 of the “subport” option in Portfiles, which is fine and should happen.
 This one is about whether to call out the use of “`subport=foo`” on the
 command line, which I don’t think we should do. (We don’t explain how to
 set any other command-line option/value pairs, either, because using them
 is not supported.)

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Ticket URL: <https://trac.macports.org/ticket/39939#comment:4>
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