[MacPorts] #16118: Add documentation for various port commands (was: Add documentation for `port gohome`)

[MacPorts]

#16118: Add documentation for various port commands
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  Reporter:  raimue at macports.org  |       Owner:  markd at macports.org     
      Type:  enhancement          |      Status:  new                    
  Priority:  Normal               |   Milestone:  Website & Documentation
 Component:  guide                |     Version:                         
Resolution:                       |    Keywords:  gohome documentation   
      Port:                       |  
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Comment(by blb at macports.org):

 Updating the patch to include raimue@'s improved description for {{{port
 url}}} (though the URL is the path for the port, not the Portfile itself),
 and added an initial section for {{{port mirror}}} which probably needs
 some improvement.

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Ticket URL: <http://trac.macports.org/ticket/16118#comment:6>
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