Fix formatting issue for docstrings without param list

Fixes bug occured in Ceilometer V2 API doc:
http://docs.openstack.org/developer/ceilometer/webapi/v2.html#capabilities

The patch adds a blank line after the doc string, if no parameter is
specified and the last line of the description contains other characters
than whitspace. With this blank line the 'Return type' will be separated
from the doc string, so it will be formatted correctly.