Use text instead of string for verification state

String type columns are no longer allowed. So update the documentation
to use text type instead and set the limit according to the guidelines.

doc/development/geo/framework.md

@@ -360,11 +360,33 @@ Widgets should now be replicated by Geo!
      DOWNTIME = false
 
      def change
-       add_column :widgets, :verification_retry_at, :datetime_with_timezone
-       add_column :widgets, :verified_at, :datetime_with_timezone
-       add_column :widgets, :verification_checksum, :binary, using: 'verification_checksum::bytea'
-       add_column :widgets, :verification_failure, :string
-       add_column :widgets, :verification_retry_count, :integer
+       change_table(:widgets) do |t|
+         t.integer :verification_retry_count, limit: 2
+         t.column :verification_retry_at, :datetime_with_timezone
+         t.column :verified_at, :datetime_with_timezone
+         t.binary :verification_checksum, using: 'verification_checksum::bytea'
+
+         # rubocop:disable Migration/AddLimitToTextColumns
+         t.text :verification_failure
+         # rubocop:enable Migration/AddLimitToTextColumns
+       end
+     end
+   end
+   ```
+
+   Adding a `text` column also [requires](../database/strings_and_the_text_data_type.md#add-a-text-column-to-an-existing-table)
+   setting a limit:
+
+   ```ruby
+   # frozen_string_literal: true
+
+   class AddVerificationFailureLimitToWidgets < ActiveRecord::Migration[6.0]
+     DOWNTIME = false
+
+     disable_ddl_transaction!
+
+     def change
+       add_text_limit :widgets, :verification_failure, 255
      end
    end
    ```