revises RDoc of AR::Base#==

activerecord/lib/active_record/base.rb

@@ -1582,7 +1582,15 @@ MSG
         self.class.columns_hash[name.to_s]
       end
 
-      # Returns true if the +comparison_object+ is the same object, or is of the same type and has the same id.
+      # Returns true if +comparison_object+ is the same exact object, or +comparison_object+ 
+      # is of the same type and +self+ has an ID and it is equal to +comparison_object.id+.
+      #
+      # Note that new records are different from any other record by definition, unless the
+      # other record is the receiver itself. Besides, if you fetch existing records with
+      # +select+ and leave the ID out, you're on your own, this predicate will return false.
+      #
+      # Note also that destroying a record preserves its ID in the model instance, so deleted
+      # models are still comparable.
       def ==(comparison_object)
         comparison_object.equal?(self) ||
           comparison_object.instance_of?(self.class) &&