Better testing and documentation on columns with _id suffix

What does this MR do?

We consider it bad practice to use the _id suffix for columns that are not a foreign key to another table, because Ruby on Rails might expect this.

So we make sure tests will fail on these columns without FK. And document the recommended _xid suffix to IDs that are external.

Closes #212218 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content