[MacPorts] #38440: document add_users

[MacPorts]

#38440: document add_users
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  Reporter:  aronnax@…    |      Owner:  markd@…
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  guide        |    Version:  2.1.3
Resolution:               |   Keywords:
      Port:               |
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Comment (by jmr@…):

 It is documented in portfile.7, so you could copy from there to the guide.

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Ticket URL: <https://trac.macports.org/ticket/38440#comment:2>
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