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.
 
 
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