Fix inaccurate timeout docstring

The docstring on the `timeout` config key implies that if no value for
a timeout is provided, initilization will block forever. This disagrees
with the `DeciderContextFactory` docstring (`"defaults to not blocking"`) and the underlying Baseplate `FileWatcher` ([source](https://github.com/reddit/baseplate.py/blob/f29bd1ce0f1ec4962f65ecd5a2b016b1cd4fd5ac/baseplate/lib/file_watcher.py#L82)).

We've also seen this not block in production. Logs can be provided if
needed.

Author: cshoe

reddit_decider/__init__.py

@@ -1165,7 +1165,7 @@ def decider_client_from_config(
             experiment configuration fetcher daemon.
         ``timeout`` (optional)
             The time that we should wait for the file specified by ``path`` to
-            exist.  Defaults to `None` which is `infinite`.
+            exist.  Defaults to `None` which is not blocking.
         ``backoff`` (optional)
             The base amount of time for exponential backoff when trying to find the
             experiments config file. Defaults to no backoff between tries.