Public SecureRandom.alphanumeric method does not appear in rdoc generated documentation
private def keywords for methods above the
SecureRandom.alphanumeric definition (method via #10849) appear to have caused rdoc generation to not output anything for the method.
See http://ruby-doc.org/stdlib-2.5.0/libdoc/securerandom/rdoc/SecureRandom.html for evidence of it being absent from rdoc generated documentation.
Speaking with one of the ruby-doc maintainers, he was the one who suggested this was the root cause of the missing entry.
This issue & patch is also reflected on GitHub: https://github.com/ruby/ruby/pull/1814