new(tests): EOF - EIP-6206: clarify "non-returning instruction"

[pdobacz]

🗒️ Description

Recently some confusion was reported whether what are "non-returning instructions" mentioned in the test. It looks like a mental shortcut, and can also raise questions like "is RETURNCONTRACT or RETURN one?". This clarifies this and also adds some coverage for such instructions (which are basically "terminating but not RETF or JUMPF-to-returning").

Also fixes a typo in that file's name.

🔗 Related Issues

NA

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: All converted JSON/YML tests from ethereum/tests have been added to converted-ethereum-tests.txt.
• Tests: A PR with removal of converted JSON/YML tests from ethereum/tests have been opened.
• Tests: Included the type and version of evm t8n tool used to locally execute test cases: e.g., ref with commit hash or geth 1.13.1-stable-3f40e65.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.

[pdobacz]

I'll go ahead and merge here as there are no fw changes involved