pylibmc: add clarifying comments

Author: clutchski

ddtrace/contrib/pylibmc/addrs.py

@@ -3,6 +3,8 @@
 translate_server_specs = None
 
 try:
+    # NOTE: we rely on an undocumented method to parse addresses,
+    # so be a bit defensive and don't assume it exists.
     from pylibmc.client import translate_server_specs
 except ImportError:
     pass

tests/contrib/flask_cache/test_wrapper_safety.py

@@ -228,4 +228,5 @@ def test_memcached_cache_tracing_with_a_wrong_connection(self):
         eq_(span.meta[net.TARGET_HOST], 'localhost')
         eq_(span.meta[net.TARGET_PORT], '22230')
 
-        # pylibmc raises an exception, memcached doesn't, so don't test that.
+        # the pylibmc backend raises an exception and memcached backend does
+        # not, so don't test anything about the status.