Add documentation on interpreting Trio tracebacks

Perhaps this is a general Python issue, but it's rather confusing that the last line of a nursery block appears in exception traces regardless of the line being reached. Seen with Python 3.6 / Trio 0.9.0:

import trio

async def async_main():
    async with trio.open_nursery() as nursery:
        @nursery.start_soon
        async def raiser():
            await trio.sleep(1)
            raise Exception('boo')

        await trio.sleep_forever()
        assert False
        print("Last line of nursery block")

try:
    trio.run(async_main)
except KeyboardInterrupt:
    pass

Traceback (most recent call last):
  File "trace_mystery.py", line 17, in <module>
    trio.run(async_main)
  File "/.../trio/_core/_run.py", line 1337, in run
    raise runner.main_task_outcome.error
  File "trace_mystery.py", line 13, in async_main
    print("Last line of nursery block")                              <--- ????
  File "/.../trio/_core/_run.py", line 397, in __aexit__
    raise combined_error_from_nursery
  File "trace_mystery.py", line 9, in raiser
    raise Exception('boo')
Exception: boo

Start with the reproduction in trace_mystery.py and compare its traceback with the referenced paths in trio/_core/_run.py. Document how to interpret the nursery traceback, including the displayed line in async_main and the raiser location, and use the example to check that the explanation matches the reported output.