doc: Add missing words to bgworker docs.

Maciek Sakrejda
Issue error on SET outside transaction block in some cases

Issue error for SET LOCAL/CONSTRAINTS/TRANSACTION outside a transaction
block, as they have no effect.

Per suggestion from Morten Hustveit
isolationtester: Allow tuples to be returned in more places

Previously, isolationtester would forbid returning tuples in
session-specific teardown (but not global teardown), as well as in
global setup.  Allow these places to return tuples, too.
Make some isolationtester specs more complete

Also, make sure they pass on all transaction isolation levels.
add multixact-no-deadlock to schedule