Fix an error for Style/Documentation

[koic]

This PR fixes an error for Style/Documentation when namespace is a variable.

m = Module.new
module n::N
  def method
  end
end

% rubocop documentation.rb --only Style/Documentation -d
(snip)

undefined method `namespace' for s(:lvar, :m):RuboCop::AST::Node
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/documentation.rb:187:in `qualify_const'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/documentation.rb:187:in `qualify_const'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/style/documentation.rb:181:in `block in identifier'

It also complements missing cbase and namespaces in offense messages. e.g.:

 class ::MyClass
-^^^^^^^^^^^^^^^ Missing top-level documentation comment for `class MyClass`.
+^^^^^^^^^^^^^^^ Missing top-level documentation comment for `class ::MyClass`.

 class self::MyClass
-^^^^^^^^^^^^^^^^^^^ Missing top-level documentation comment for `class MyClass`.
+^^^^^^^^^^^^^^^^^^^ Missing top-level documentation comment for `class self::MyClass`.

 class my_method::MyClass
-^^^^^^^^^^^^^^^^^^^^^^^^ Missing top-level documentation comment for `class MyClass`.
+^^^^^^^^^^^^^^^^^^^^^^^^ Missing top-level documentation comment for `class my_method::MyClass`.

---

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]

Looks good. Thanks!