My bet is that it is built on top of Aurora PostgreSQL. By looking at the "Limits" section (https://docs.aws.amazon.com/documentdb/latest/developerguide...), identifiers are limited to 63 characters and the same characters that PostgreSQL limits identifiers to; and a collection size limit of 32TB, coincidentally maximum PostgreSQL table size.
Edit: I can confirm: does not allow the UTF-8 null character in strings: https://docs.aws.amazon.com/documentdb/latest/developerguide... ... It is written on top of PostgreSQL.