解决方法
> gem help rdoc Usage: gem rdoc [args] [options] Options: --all Generate RDoc/RI documentation for all installed gems --[no-]rdoc Include RDoc generated documents --[no-]ri Include RI generated documents -v,--version VERSION Specify version of gem to rdoc Arguments: GEMNAME gem to generate documentation for (unless --all) Summary: Generates RDoc for pre-installed gems Defaults: --version '>= 0' --rdoc --ri