如果ri Array的时候提示Nothing known about Array, 那么rvm docs generate 之后等10分钟
就能看到了：
~ ➤ rvm docs generate
Generating ri documentation, be aware that this could take a *long* time, and depends heavily on your system resources...
( Errors will be logged to /Users/lenage/.rvm/log/ruby-1.9.3-p125/docs.log )
~ ➤ tail -f /Users/lenage/.rvm/log/ruby-1.9.3-p125/docs.log
Generating RI format into /Users/lenage/.rvm/rubies/ruby-1.9.3-p125/share/ri/1.9.1/site...
Generating Darkfish format into /Users/lenage/.rvm/docs/ruby-1.9.3-p125/rdoc..