Improve examples for LeakyConstantDeclaration cop

As mentioned here
rubocop#765 (comment),
it's not even necessary in some cases to stub a constant, a so-called
"sacrificial class" will do.
Originally mentioned article
http://blog.bitwrangler.com/2016/11/10/sacrificial-test-classes.html in
this comment rubocop#197 (comment)

lib/rubocop/cop/rspec/leaky_constant_declaration.rb

@@ -23,35 +23,51 @@ module RSpec
       #   # bad
       #   describe SomeClass do
       #     OtherClass = Struct.new
-      #     CONSTANT_HERE = 'is also denied'
+      #     CONSTANT_HERE = 'I leak into global namespace'
       #   end
       #
       #   # good
       #   describe SomeClass do
       #     before do
       #       stub_const('OtherClass', Struct.new)
-      #       stub_const('CONSTANT_HERE', 'is also denied')
+      #       stub_const('CONSTANT_HERE', 'I am undefined after the example')
       #     end
       #   end
       #
       # @example
       #   # bad
       #   describe SomeClass do
-      #     class OtherClass < described_class
-      #       def do_something
+      #     class FooClass < described_class
+      #       def double_that
+      #         some_base_method * 2
       #       end
       #     end
+      #
+      #     it { expect(FooClass.new.double_that).to eq(4) }
       #   end
       #
-      #   # good
+      #   # good - sacrificial class, no constant needs to be defined
+      #   let(:foo_class) do
+      #     Class.new(described_class) do
+      #       def double_that
+      #         some_base_method * 2
+      #       end
+      #     end
+      #
+      #     it { expect(foo_class.new.double_that).to eq(4) }
+      #   end
+      #
+      #   # good - constant is stubbed
       #   describe SomeClass do
       #     before do
       #       fake_class = Class.new(described_class) do
       #                      def do_something
       #                      end
       #                    end
-      #       stub_const('OtherClass', fake_class)
+      #       stub_const('FooClass', fake_class)
       #     end
+      #
+      #     it { expect(FooClass.new.double_that).to eq(4) }
       #   end
       #
       # @example

manual/cops_rspec.md

@@ -1506,35 +1506,51 @@ namespace name clashes.
 # bad
 describe SomeClass do
   OtherClass = Struct.new
-  CONSTANT_HERE = 'is also denied'
+  CONSTANT_HERE = 'I leak into global namespace'
 end
 
 # good
 describe SomeClass do
   before do
     stub_const('OtherClass', Struct.new)
-    stub_const('CONSTANT_HERE', 'is also denied')
+    stub_const('CONSTANT_HERE', 'I am undefined after the example')
   end
 end
 ```
 ```ruby
 # bad
 describe SomeClass do
-  class OtherClass < described_class
-    def do_something
+  class FooClass < described_class
+    def double_that
+      some_base_method * 2
     end
   end
+
+  it { expect(FooClass.new.double_that).to eq(4) }
 end
 
-# good
+# good - sacrificial class, no constant needs to be defined
+let(:foo_class) do
+  Class.new(described_class) do
+    def double_that
+      some_base_method * 2
+    end
+  end
+
+  it { expect(foo_class.new.double_that).to eq(4) }
+end
+
+# good - constant is stubbed
 describe SomeClass do
   before do
     fake_class = Class.new(described_class) do
                    def do_something
                    end
                  end
-    stub_const('OtherClass', fake_class)
+    stub_const('FooClass', fake_class)
   end
+
+  it { expect(FooClass.new.double_that).to eq(4) }
 end
 ```
 ```ruby