[MacPorts] #58338: cmake variant: rename +docs to +doc

[MacPorts]

#58338: cmake variant: rename +docs to +doc
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 Reporter:  blair    |      Owner:  michaelld
     Type:  request  |     Status:  assigned
 Priority:  Normal   |  Milestone:
Component:  ports    |    Version:
 Keywords:           |       Port:  cmake
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 It appears that +doc is more common than +doc (at least in my list of
 installed packages), as I have +doc in my
 ${prefix}/etc/macports/variants.conf, so suggest cmake switches over.

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Ticket URL: <https://trac.macports.org/ticket/58338>
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