Update `spec.md`: TSV entity references

[joeflack4]

Does not address an issue, but relevant context is: here

Changes

• Add: Subsection in TSV subsection about entity reference validity.

Checks

• [x] docs/ have been added/updated if necessary

Not applicable:

• [x] make test has been run locally
• [x] tests have been added/updated (if applicable)
• [x] CHANGELOG.md has been updated.