[MacPorts] #68506: Document extract.rename

[MacPorts]

#68506: Document extract.rename
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 Reporter:  chrstphrchvz  |      Owner:  (none)
     Type:  enhancement   |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  guide         |    Version:  2.8.1
 Keywords:                |       Port:
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 The `extract.rename` option from MacPorts 2.8.1 ([8ae0d735/macports-base])
 has yet to be mentioned in the guide.

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Ticket URL: <https://trac.macports.org/ticket/68506>
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