I didn't even know there was an add_runtime_dependency.  I've only ever seen add_dependency.  I did a double check of some random popular gems, namely activesupport, aws-sdk, and ancestry, and all of them use `add_dependency`, so I think that's the popular choice.

As far as documentation goes, from googling your best bets are usually ruby-doc.org, apidock.com, or rubydoc.info.
http://ruby-doc.org/stdlib-1.9.3/libdoc/rubygems/rdoc/Gem/Specification.html#method-i-add_dependency