Fix an error for Style/Documentation

[koic]

This PR fixes the following error for Style/Documentation when using a cbase class.

% cat example.rb
class ::MyClass
  def method
  end
end

% bundle exec rubocop --only Style/Documentation -d
(snip)

An error occurred while Style/Documentation cop was inspecting
/Users/koic/src/github.com/koic/rubocop-issues/bar/example.rb:1:0.
undefined method `namespace' for s(:cbase):RuboCop::AST::Node
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/documentation.rb:181:in `qualify_const'

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

Thanks!