[Fix #11504] Allow initialize method in Style/DocumentationMethod

Fixes #11504.

This PR makes to allow `initialize` method in `Style/DocumentationMethod`
because `initialize` is a special method called from `new`.
In some languages they are called constructor to distinguish it from method.

So it would be different from the following intent that this cop wants to detect.

> Checks for missing documentation comment for public methods.
> It can optionally be configured to also require documentation for
> non-public methods.

changelog/change_allow_initialize_method_in_sytle_documentation_method.md

@@ -0,0 +1 @@
+* [#11504](https://github.com/rubocop/rubocop/issues/11504): Allow `initialize` method in `Style/DocumentationMethod`. ([@koic][])

lib/rubocop/cop/style/documentation_method.rb

@@ -7,6 +7,10 @@ module Style
       # It can optionally be configured to also require documentation for
       # non-public methods.
       #
+      # NOTE: This cop allows `initialize` method because `initialize` is
+      # a special method called from `new`. In some programming languages
+      # they are called constructor to distinguish it from method.
+      #
       # @example
       #
       #   # bad
@@ -103,6 +107,8 @@ class DocumentationMethod < Base
         PATTERN
 
         def on_def(node)
+          return if node.method?(:initialize)
+
           parent = node.parent
           module_function_node?(parent) ? check(parent) : check(node)
         end

spec/rubocop/cop/style/documentation_method_spec.rb

@@ -52,6 +52,15 @@ def foo
         end
       end
 
+      context 'when `initialize` method' do
+        it 'does not register an offense' do
+          expect_no_offenses(<<~RUBY)
+            def initialize
+            end
+          RUBY
+        end
+      end
+
       context 'when method is private' do
         it 'does not register an offense' do
           expect_no_offenses(<<~RUBY)