The relevant error from your log seems to be:
:info:destroot WARNING: builder-2.1.2 has an invalid nil value for @cert_chain
:info:destroot ERROR: While generating documentation for builder-2.1.2
:info:destroot ... MESSAGE: Unhandled special: Special: type=17, text="<!-- HI -->"
:info:destroot ... RDOC args: --ri --op /opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_ports_ruby_rb-builder/rb-builder/work/destroot/opt/local/lib/ruby/gems/1.8/doc/builder-2.1.2/ri --title Builder -- Easy XML Building --main README --line-numbers lib CHANGES Rakefile README doc/releases/builder-1.2.4.rdoc doc/releases/builder-2.0.0.rdoc doc/releases/builder-2.1.1.rdoc --title builder-2.1.2 Documentation --quiet
I'm not familiar with ruby though and I don't know what this means or what to do about it.