[2843] revise docstring of dns.Counters()

src/lib/python/isc/statistics/dns.py

@@ -178,9 +178,10 @@ class Counters(counters.Counters):
     _statistics = _Statistics()
 
     def __init__(self, spec_file_name=None):
-        """If the spec file has 'zones', it obtains a list of counter
-        names under it when initiating.  For behaviors other than
-        this, see isc.statistics.counters.Counters.__init__.__doc__."""
+        """If the item `zones` is defined in the spec file, it obtains a
+        list of counter names under it when initiating.  For behaviors
+        other than this, see
+        isc.statistics.counters.Counters.__init__.__doc__."""
         counters.Counters.__init__(self, spec_file_name)
         if self._perzone_prefix in \
                 isc.config.spec_name_list(self._statistics._spec):