Add descriptions for the RSpec/EmptyExampleGroup and RSpec/ExpectOutput cops

As requested in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11497#note_30049512 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11500#note_30044276.

.rubocop.yml

@@ -969,7 +969,7 @@ RSpec/DescribeSymbol:
 RSpec/DescribedClass:
   Enabled: true
-# Configuration parameters: CustomIncludeMethods.
+# Checks if an example group does not include any tests.
 RSpec/EmptyExampleGroup:
   Enabled: true
   CustomIncludeMethods:
@@ -993,6 +993,7 @@ RSpec/ExampleWording:
 RSpec/ExpectActual:
   Enabled: true
+# Checks for opportunities to use `expect { … }.to output`.
 RSpec/ExpectOutput:
   Enabled: true