[MacPorts] #50023: Clarification of variant specifiers in section 3.1.10 of guide

[MacPorts]

#50023: Clarification of variant specifiers in section 3.1.10 of guide
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  Reporter:  alexreg@…   |      Owner:  markd@…
      Type:  submission  |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  guide       |    Version:  2.3.4
Resolution:              |   Keywords:
      Port:              |
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Comment (by cal@…):

 That change removes the explanation of negative variant output, e.g. in
 `port variants nginx -spdy`. Please restore it.

 Additionally, we prefer unified patches, since they are easier to review
 and apply when the file has changed meanwhile. Additionally, the using.xml
 file uses spaces for indentation, please keep it that way.

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Ticket URL: <https://trac.macports.org/ticket/50023#comment:2>
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