Mention invalid null byte sequence

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/datatype-character.html
Description:

I discovered accidentally that PostgreSQL doesn't accept null byte in text
type. It seems that Oracle does (see
https://www.postgresql.org/message-id/de752e01-f36c-821e-9181-cfba78c0fbc8%40propaas.com)
and SQLite does it too.

So it should written in the character type that null byte are not accepted,
it would make like easier to migrate to PostgreSQL :)