Hello,

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Cygportfile:
- https://cygwin.com/cgit/cygwin-packages/playground/tree/?h=ruby-rdoc

Packages:
- https://yacp.osdn.jp/ita/ruby-rdoc/

> Therefore, it is necessary to update rdoc first, but packaging
> rdoc-6.5.0 also fails for the same reason, so it needs to be updated
> by other means besides scallywag:
> - Create a package using ruby-2.6.4.
> - Create a package using rdoc-6.5.0 installed in a different location
> with gem install.

I created a package with the latter.