Simplify gemrc nodoc

Per https://stackoverflow.com/questions/1381725/how-to-make-no-ri-no-rdoc-the-default-for-gem-install#comment22636260_7662245
this is possible since rubygems 2.0.0 from 2012.

I updated the comment to indicate this is only for the `gem` command,
bundler already doesn't do this by default.

It works:
```
$ gem install rubocop -v 1.69.0
Fetching rubocop-1.69.0.gem
Successfully installed rubocop-1.69.0
1 gem installed
$ gem update rubocop
Updating installed gems
Updating rubocop
Fetching rubocop-1.69.1.gem
Successfully installed rubocop-1.69.1
Gems updated: rubocop
```

Author: Earlopain