It seems that the documents generated by tbls confuse the terms "relation" and "relationship".
Terminology
In SQL terminology, a “table” corresponds to a "relation" in relational database terminology.
In a relational database, a relation is a set of tuples that have the same attributes.
— https://en.wikipedia.org/wiki/Relational_database
"Relationships" are a logical connection between different tables.
Relationships are a logical connection between different tables (entities), established on the basis of interaction among these tables.
— https://en.wikipedia.org/wiki/Relational_database
Current behavior
For example, tbls generates ER-diagram in a section titled "relations".
Expected behavior
the section title should be "relationship" instead of "relation".
because ER-diagram stands for Entity-Relationship diagram, not Entity-Relation diagram.