Well, as I said, that's a text value with a byte limit. The documentation should definitely mention something like "with the default database encoding".
I'm not sure why you think adding such wordage is needed. Bytes is bytes. Your application should be checking the number of bytes in the string sent to NOTIFY.
On a related note, if you find yourself approaching the payload limits, you might want to rethink the way you are using LISTEN/NOTIFY.