1 files changed, 33 insertions, 3 deletions
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index 76ee5c0c48..10a9f24c9a 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -85,13 +85,14 @@ Rails 6.1. You are encouraged to enable `config.force_ssl` to enforce HTTPS
 connections throughout your application. If you need to exempt certain endpoints
 from redirection, you can use `config.ssl_options` to configure that behavior.
 
-### Purpose in signed or encrypted cookie is now embedded within cookies
+### Purpose and expiry metadata is now embedded inside signed and encrypted cookies for increased security
+
+To improve security, Rails embeds the purpose and expiry metadata inside encrypted or signed cookies value.
 
-To improve security, Rails embeds the purpose information in encrypted or signed cookies value.
 Rails can then thwart attacks that attempt to copy the signed/encrypted value
 of a cookie and use it as the value of another cookie.
 
-This new embed information make those cookies incompatible with versions of Rails older than 6.0.
+This new embed metadata make those cookies incompatible with versions of Rails older than 6.0.
 
 If you require your cookies to be read by Rails 5.2 and older, or you are still validating your 6.0 deploy and want
 to be able to rollback set
@@ -281,6 +282,35 @@ won't work, child objects like `Hotel::Pricing` won't be found.
 
 This restriction only applies to explicit namespaces. Classes and modules not defining a namespace can be defined using those idioms.
 
+#### One file, one constant (at the same top-level)
+
+In `classic` mode you could technically define several constants at the same top-level and have them all reloaded. For example, given
+
+```ruby
+# app/models/foo.rb
+
+class Foo
+end
+
+class Bar
+end
+```
+
+while `Bar` could not be autoloaded, autoloading `Foo` would mark `Bar` as autoloaded too. This is not the case in `zeitwerk` mode, you need to move `Bar` to its own file `bar.rb`. One file, one constant.
+
+This affects only to constants at the same top-level as in the example above. Inner classes and modules are fine. For example, consider
+
+```ruby
+# app/models/foo.rb
+
+class Foo
+  class InnerClass
+  end
+end
+```
+
+If the application reloads `Foo`, it will reload `Foo::InnerClass` too.
+
 #### Spring and the `test` Environment
 
 Spring reloads the application code if something changes. In the `test` environment you need to enable reloading for that to work: