Add a comment about why we are taking a lock when terminating a builder machine.

buildman/manager/ephemeral.py

@@ -105,6 +105,8 @@ class EphemeralBuilderManager(BaseManager):
       if 'builder_id' in job_metadata:
         builder_id = job_metadata['builder_id']
 
+        # Before we delete the build node, we take a lock to make sure that only one manager
+        # can terminate the node.
         try:
           lock_key = self._etcd_lock_key(builder_id)
           yield From(self._etcd_client.write(lock_key, '', prevExist=False, ttl=self.setup_time()))