API Documentation
Capistrano, like all most gems ships its RDoc packaged within, to access this, the easiest most convenient way is to use your own local gem server - this can be improved and made permanent using Jesse Newland's excellent sinatra-rubygems.
If you don't want to install that, or you'd rather just review it online, you can find it here (amongst other places):
API Style Documentation
Dependencies
The documentation for Capistrano's Net::SCP, Net::SSH and other dependencies can be found on the projects rubyforge page.