Add comments and use list comprehensions

Per PR review, use list comprehensions as they are prefered in py3 over
use of filter.

Add comments to describe the building of the zone tree.

octodns/zone.py

@@ -69,8 +69,7 @@ class Zone(object):
 
         name = record.name
 
-        if not lenient and any(map(lambda sz: name.endswith(sz),
-                                   self.sub_zones)):
+        if not lenient and any((name.endswith(sz) for sz in self.sub_zones)):
             if name not in self.sub_zones:
                 # it's a record for something under a sub-zone
                 raise SubzoneRecordException(f'Record {record.fqdn} is under '