fixup! Note to use :config in custom cop specs

Author: pirj

docs/modules/ROOT/pages/upgrade_to_version_2.adoc

@@ -10,6 +10,7 @@ In version 2.x:
  - cop departments are nested for cops with a department that doesn’t match the extension name (`Capybara`, `FactoryBot`, `Rails`)
  - `AllCops/RSpec/Patterns`/`AllCops/FactoryBot/Patterns` options are removed
  - Calling `super` from `on_new_investigation` defined in a cop is mandatory now
+ - Avoid explicit definition of `cop` as `described_class.new`
 
 [discrete]
 === Adjust the configuration of `RSpec/EmptyExampleGroup`
@@ -241,6 +242,30 @@ end
 
 https://github.com/rubocop-hq/rubocop-rspec/pull/956[Pull request with more examples].
 
+=== Use `:config` RSpec metadata in cop specs
+
+`:config` metadata should be added to the top-level example group of your cop spec.
+Doing otherwise will not pass configuration to the cop, and dynamic RSpec DSL matchers might not work.
+
+[source,ruby]
+----
+# Before
+RSpec.describe 'MyMightyCop' do
+  let(:cop) { described_class.new }
+  # ...
+end
+
+# After
+RSpec.describe 'MyMightyCop', :config do
+  # `cop` is defined for you by RuboCop's shared context that is included
+  # to example groups with :config metadata
+
+  # ...
+end
+----
+
+https://github.com/rubocop-hq/rubocop/blob/51ff1d7e29c985732fe129082c98d66c531a2611/lib/rubocop/rspec/shared_contexts.rb#L56[RuboCop takes care of defining everything for your cop specs].
+
 === Conform with RuboCop API Changes
 
 The parent project, RuboCop, has API changes.