kcsan: Update API documentation in kcsan-checks.h

Update the API documentation for ASSERT_EXCLUSIVE_* macros and make them
generate readable documentation for the code examples.

All @variable short summaries were missing ':', which was updated for
the whole file.

Tested with "make htmldocs".

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>