Clarify that you can raise exceptions without arguments

Whilst `MyCustomError.new` violates this Rubocop rule, the documentation implies the only way of fixing it is to pass an argument representing the exception message, whereas actually if your custom exception has a default message, it's quite possible you just want to use the default message. This can be achieved without violating this Rubocop rule, by omitting the call to `.new`.

lib/rubocop/cop/style/raise_args.rb

@@ -23,6 +23,7 @@ module Style
       #   # good
       #   raise StandardError, 'message'
       #   fail 'message'
+      #   raise MyCustomError
       #   raise MyCustomError.new(arg1, arg2, arg3)
       #   raise MyKwArgError.new(key1: val1, key2: val2)
       #
@@ -37,6 +38,7 @@ module Style
       #
       #   # good
       #   raise StandardError.new('message')
+      #   raise MyCustomError
       #   raise MyCustomError.new(arg1, arg2, arg3)
       #   fail 'message'
       class RaiseArgs < Base