Refactor ExplainFormatter

spec/ameba/formatter/explain_formatter_spec.cr

@@ -59,7 +59,7 @@ module Ameba
         source = Source.new "a = 42", "source.cr"
         output = explanation(source)
         output.should contain "DETAILED DESCRIPTION"
-        output.should contain "TO BE DONE..."
+        output.should contain "Rule extended description"
       end
 
       it "writes nothing if location not found" do

spec/spec_helper.cr

@@ -24,6 +24,7 @@ module Ameba
     end
   end
 
+  # Rule extended description
   class ErrorRule < Rule::Base
     properties do
       description "Always adds an error at 1:1"