Add missing YARD documentation

These comments were the only missing docs flagged by yardstick

lib/mutant/ast/named_children.rb

@@ -79,6 +79,8 @@ module Mutant
         # Define private method
         #
         # @param [Symbol] name
+        #
+        # @return [undefined]
         def define_private_method(name, &block)
           define_method(name, &block)
           private(name)

lib/mutant/reporter/sequence.rb

@@ -13,6 +13,9 @@ module Mutant
         end
       end
 
+      # Minimum reporter delay
+      #
+      # @return [Float]
       def delay
         reporters.map(&:delay).min
       end