Document earliest date for email domain first_seen field

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: horgh

lib/minfraud/model/email_domain.rb

@@ -16,9 +16,10 @@ class EmailDomain < Abstract
       # @return [String, nil]
       attr_reader :classification
 
-      # A date string (e.g. 2017-04-24) to identify the date an email domain
+      # A date string (e.g. 2019-01-01) to identify the date an email domain
       # was first seen by MaxMind. This is expressed using the ISO 8601 date
-      # format.
+      # format YYYY-MM-DD. The earliest date that may be returned is January
+      # 1, 2019.
       #
       # @return [String, nil]
       attr_reader :first_seen