This is a follow up to b129511050
which changes the code block in the docs to python and also
cleans up some of the NumInstancesFilter documentation, specifically
removing wording around "running" instances because the filter
does not care about state, only the number of instances reported
against a host - they could be stopped/paused/etc.
Change-Id: I7cdf79f21415fbf799fa5ad586f1b8063afed272