[DOC] Don't document gemspec files
Since https://github.com/ruby/ruby/commit/cf711863cbe0b81be5ff0adfb64936137995cc4e, gemspecs are included because the default exclude is overwritten
[DOC] Don't document gemspec files
Since https://github.com/ruby/ruby/commit/cf711863cbe0b81be5ff0adfb64936137995cc4e, gemspecs are included because the default exclude is overwritten