Denote what happens when no exceptions are passed in
Denote what happens when no exceptions are passed in

When no exceptions are passed in and the exception
tuple is empty this appears to catch no exceptions so
document that this happens.

  >>> excps = ()
  >>> try:
  ...   raise IOError("I am broken")
  ... except excps:
  ...   print("captured")
  ...
  Traceback (most recent call last):
    File "<stdin>", line 2, in <module>
  IOError: I am broken

Change-Id: I141ffd53b8f864bb0502d2e022264ea3145213ff

openstack/oslo.service,poznyakandrey/oslo.service