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