I’ve yet to meet a Rails developer who either looks at or even needs the local gem documentation. All it does is slow down the installation of gems.
You could use the relevant switches as follows…
gem install nokogiri --no-rdoc --no-ri
…or you could simply add the following line to the .gemrc file in your home directory:
gem: --no-rdoc --no-ri
Then you can freely install gems without using swtiches and without any time-consuming documetation downloads:
gem install nokogiri