Documenting "virtual" implicit entities

Some data models have "missing" implicit entities modeled just as a value/code in columns of existing tables. To explain it, let's have a look at the tables below. They use the document_type column, representing the type of the document represented by the code, but the document type table is nowhere to be found.

In such cases, document this entity and the relationships using a manual table that would represent that entity and then define relationships between the real and manual tables.