Ensure on_rotation appears in RDoc [ci-skip]

When a `:method:` doc is immediately followed by the `private` keyword,
RDoc will hide that doc as if it were a private method.

To ensure that `ActiveSupport::MessageEncryptors#on_rotation` and
`ActiveSupport::MessageVerifiers#on_rotation` both appear in the
rendered docs, this commit adds a delimiter comment before each
`private` keyword.

activesupport/lib/active_support/message_encryptors.rb

@@ -130,6 +130,7 @@ class MessageEncryptors < Messages::RotationCoordinator
     # indicate whether old option sets are still in use or can be removed from
     # rotation.
 
+    ##
     private
       def build(salt, secret_generator:, secret_generator_options:, **options)
         secret_length = MessageEncryptor.key_len(*options[:cipher])

activesupport/lib/active_support/message_verifiers.rb

@@ -126,6 +126,7 @@ class MessageVerifiers < Messages::RotationCoordinator
     # indicate whether old option sets are still in use or can be removed from
     # rotation.
 
+    ##
     private
       def build(salt, secret_generator:, secret_generator_options:, **options)
         MessageVerifier.new(secret_generator.call(salt, **secret_generator_options), **options)