Improve deprecation message with example of how to remove it

The deprecation of `devise_error_messages!` wasn't super clear on what
was happening and how to get rid of the message, not it has a more
detailed explanation with an example of what to look for and what to
replace it with.

Closes #5257.

app/helpers/devise_helper.rb

@@ -1,14 +1,26 @@
 # frozen_string_literal: true
 
 module DeviseHelper
-  # Retain this method for backwards compatibility, deprecated in favour of modifying the
-  # devise/shared/error_messages partial
+  # Retain this method for backwards compatibility, deprecated in favor of modifying the
+  # devise/shared/error_messages partial.
   def devise_error_messages!
     ActiveSupport::Deprecation.warn <<-DEPRECATION.strip_heredoc
-      [Devise] `DeviseHelper.devise_error_messages!`
-      is deprecated and it will be removed in the next major version.
-      To customize the errors styles please run `rails g devise:views` and modify the
-      `devise/shared/error_messages` partial.
+      [Devise] `DeviseHelper#devise_error_messages!` is deprecated and will be
+      removed in the next major version.
+
+      Devise now uses a partial under "devise/shared/error_messages" to display
+      error messages by default, and make them easier to customize. Update your
+      views changing calls from:
+
+          <%= devise_error_messages! %>
+
+      to:
+
+          <%= render "devise/shared/error_messages", resource: resource %>
+
+      To start customizing how errors are displayed, you can copy the partial
+      from devise to your `app/views` folder. Alternatively, you can run
+      `rails g devise:views` which will copy all of them again to your app.
     DEPRECATION
 
     return "" if resource.errors.empty?