Update comment for how secret key is calculated

This updates the comment to reflect how the secret key is generated
since 4c743587

Fixes #35717

railties/lib/rails/application.rb

@@ -409,7 +409,8 @@ def secrets
     # The secret_key_base is used as the input secret to the application's key generator, which in turn
     # is used to create all MessageVerifiers/MessageEncryptors, including the ones that sign and encrypt cookies.
     #
-    # In test and development, this is simply derived as a MD5 hash of the application's name.
+    # In development and test, this is randomly generated and stored in a
+    # temporary file in <tt>tmp/development_secret.txt</tt>.
     #
     # In all other environments, we look for it first in ENV["SECRET_KEY_BASE"],
     # then credentials.secret_key_base, and finally secrets.secret_key_base. For most applications,