Mention block form of Struct.new in Style/StructInheritance

As the documentation for `Struct` recommends using the block
form over inheriting, I think it is worth mentioning it.

> If a block is given it will be evaluated in the context of
StructClass, passing the created class as a parameter

> This is the recommended way to customize a struct.

Adjust examples in the documentation and add a test which uses
the block form.

lib/rubocop/cop/style/struct_inheritance.rb

@@ -8,12 +8,20 @@ module Style
       # @example
       #   # bad
       #   class Person < Struct.new(:first_name, :last_name)
+      #     def age
+      #       42
+      #     end
       #   end
       #
       #   # good
-      #   Person = Struct.new(:first_name, :last_name)
+      #   Person = Struct.new(:first_name, :last_name) do
+      #     def age
+      #       42
+      #     end
+      #   end
       class StructInheritance < Cop
-        MSG = "Don't extend an instance initialized by `Struct.new`.".freeze
+        MSG = "Don't extend an instance initialized by `Struct.new`. " \
+              'Use a block to customize the struct.'.freeze
 
         def on_class(node)
           _name, superclass, _body = *node

manual/cops_style.md

@@ -5954,10 +5954,17 @@ This cop checks for inheritance from Struct.new.
 ```ruby
 # bad
 class Person < Struct.new(:first_name, :last_name)
+  def age
+    42
+  end
 end
 
 # good
-Person = Struct.new(:first_name, :last_name)
+Person = Struct.new(:first_name, :last_name) do
+  def age
+    42
+  end
+end
 ```
 
 ### References

spec/rubocop/cop/style/struct_inheritance_spec.rb

@@ -6,15 +6,15 @@
   it 'registers an offense when extending instance of Struct' do
     expect_offense(<<-RUBY.strip_indent)
       class Person < Struct.new(:first_name, :last_name)
-                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Don't extend an instance initialized by `Struct.new`.
+                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Don't extend an instance initialized by `Struct.new`. Use a block to customize the struct.
       end
     RUBY
   end
 
   it 'registers an offense when extending instance of Struct with do ... end' do
     expect_offense(<<-RUBY.strip_indent)
       class Person < Struct.new(:first_name, :last_name) do end
-                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Don't extend an instance initialized by `Struct.new`.
+                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Don't extend an instance initialized by `Struct.new`. Use a block to customize the struct.
       end
     RUBY
   end
@@ -36,4 +36,14 @@ class Person < DelegateClass(Animal)
   it 'accepts assignment to Struct.new' do
     expect_no_offenses('Person = Struct.new(:first_name, :last_name)')
   end
+
+  it 'accepts assignment to block form of Struct.new' do
+    expect_no_offenses(<<-RUBY.strip_indent)
+      Person = Struct.new(:first_name, :last_name) do
+        def age
+          42
+        end
+      end
+    RUBY
+  end
 end