If you need to install your gems without extra documentation, use flags –no-rdoc and –no-ri
Example:
$ gem install --no-rdoc --no-ri rails
Edit:
You can automate this with the following in your .gemrc :
gem: --no-ri --no-rdoc
It is said that using these flags on every gem command can break things, so the alternate configuration is:
install: --no-rdoc --no-ri
update: --no-rdoc --no-ri