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

src/docs/userguide/remarkup.diviner

@@ -34,7 +34,7 @@ This produces headers like the ones in this document.
 
 Make **lists** by indenting two spaces and beginning each item with a "-":
 
-  lang=demo
+  lang=text
   - milk
   - eggs
   - bread
@@ -51,7 +51,7 @@ Make **code blocks** by indenting two spaces:
 
 You can specify a language for syntax highlighting with "lang=xxx":
 
-  lang=demo
+  lang=text
   lang=html
   <a href="#">...</a>
 
@@ -64,7 +64,7 @@ available (in most cases, this means you need to configure Pygments):
 You can also use a "COUNTEREXAMPLE" header to show that a block of code is
 bad and shouldn't be copied:
 
-  lang=demo
+  lang=text
   COUNTEREXAMPLE
   function f() {
     global $$variable_variable;