Move comment up to the class, for both of the methods, and document on class level why we are doing this.

[ci skip]

activerecord/lib/active_record/locking/optimistic.rb

@@ -184,9 +184,12 @@ def inherited(subclass)
       end
     end
 
+
+    # In de/serialize we change `nil` to 0, so that we can allow passing
+    # `nil` values to `lock_version`, and not result in `ActiveRecord::StaleObjectError`
+    # during update record.
     class LockingType < DelegateClass(Type::Value) # :nodoc:
       def deserialize(value)
-        # `nil` *should* be changed to 0
         super.to_i
       end