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 1') to a test and verified it throws.
• Read the documentation.

Reviewed By: edward
Reviewers: edward, jungejason, tuomaspelkonen, aran
CC: aran, edward
Differential Revision: 773