Add comments to set_ttl method

To explain how it's supposed to work and why it manupulates with the
cache of `Cluster` object (calls `reset_cluster`)

patroni/dcs/consul.py

@@ -101,7 +101,9 @@ class Consul(AbstractDCS):
                 except Exception:
                     logger.exception("Can not destroy session %s", self._session)
             self._session = None
+            # force `watch` method to call `AbstractDCS.watch` instead of watching for leader key
             self.reset_cluster()
+            # fire up an event to wake up from `watch` and immediately run HA loop (to create the new session)
             self.event.set()
         self._ttl = ttl
 

patroni/dcs/etcd.py

@@ -218,7 +218,9 @@ class Etcd(AbstractDCS):
     def set_ttl(self, ttl):
         ttl = int(ttl)
         if self._ttl != ttl:
+            # force `watch` method to call `AbstractDCS.watch` instead of watching for leader key
             self.reset_cluster()
+            # fire up an event to wake up from `watch` and immediately run HA loop (to update TTL of leader and member)
             self.event.set()
         self._ttl = ttl