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

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

.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