Merge pull request #3704 from afeld/patch-1

clarify that the `chunk_size` is optional when streaming to a file

docs/user/quickstart.rst

@@ -184,7 +184,7 @@ streamed to a file::
 Using ``Response.iter_content`` will handle a lot of what you would otherwise
 have to handle when using ``Response.raw`` directly. When streaming a
 download, the above is the preferred and recommended way to retrieve the
-content.
+content. Note that ``chunk_size`` is optional.
 
 
 Custom Headers