With ruby < 2.6.0 / rdoc < 6.0.2, rdoc fails to parse valid ruby code,
resulting in spurious build failures. An example is asciidoctor >
2.0.15 (though 2.0.20, currently)
While attempting to build asciidoctor-2.0.20 for Fedora and RHEL+EPEL
releases, the following error fails the build on RHEL+EPEL 8:
Installing ri documentation for asciidoctor-2.0.20
Installing darkfish documentation for asciidoctor-2.0.20
ERROR: While executing gem ... (RDoc::Error)
error generating Asciidoctor/Converter/ManPageConverter.html:
no implicit conversion of nil into String (TypeError)
Resolves: rhbz#2210326
