[MacPorts] #54605: Update guide to tell user where to get "original" Portfile for patching

[MacPorts]

#54605: Update guide to tell user where to get "original" Portfile for patching
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 Reporter:  jfcaron3  |      Owner:
     Type:  defect    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  guide     |    Version:
 Keywords:            |       Port:
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 If you read:
 https://guide.macports.org/#development.patches.portfile

 It tells you how to make a copy and generate patch for a Portfile, but it
 does not tell you where to find the original Portfile.  Looking on my
 system with find /opt/local grep|portname|grep Portfile, I find two
 candidates, one somewhere in
 /opt/local//var/macports/sources/rsync.macports.org/release/tarballs/ports
 and another somewhere in /opt/local//var/macports/registry/portfiles/ and
 they appear slightly different.

 Even if this is an idiosyncrasy in my system, the guide section should
 include the default location of the Portfile to be used to generate
 patches.

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Ticket URL: <https://trac.macports.org/ticket/54605>
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