R1:29444d1df364

Add a little more unit test documentation, fail loudly when isolation prevents a
query

Summary:
- Provide an example unit test, and document it.
- Document database isolation better.
- When we issue an unsimulated query to the isolated connection, throw a
helpful message.
- Pygments is complaining about my madeup "lang=demo", change it to
"lang=text".

Test Plan:
- Ran the unit test (sanity check).
- Ran all other unit tests (verify I didn't break isolation).
- Added a queryfx(..., 'SELECT…