Add an example for AR.db_warnings_ignore [ci skip]

Author: zzak

activerecord/lib/active_record.rb

@@ -259,6 +259,9 @@ def self.db_warnings_action=(action)
   ##
   # :singleton-method: db_warnings_ignore
   # Specify allowlist of database warnings.
+  # Can be a string, regular expression, or an error code from the database.
+  #
+  #   ActiveRecord::Base.db_warnings_ignore = [/^SHOW WARNINGS` did not return the warnings/, "01000"]
   singleton_class.attr_accessor :db_warnings_ignore
   self.db_warnings_ignore = []