Add notes

lib/req.ex

@@ -89,6 +89,8 @@ defmodule Req do
       iex> Enum.map(resp.body, & &1["id"])
       [0, 1]
 
+  See `:into` option in `Req.new/1` documentation for more information on response body streaming.
+
   ## Header Names
 
   The HTTP specification requires that header names should be case-insensitive.
@@ -258,6 +260,12 @@ defmodule Req do
           `Enumerable` that receives and automatically parses messages. See module documentation
           for example usage.
 
+          If the request is sent using HTTP/1, an extra process is spawned to consume messages
+          from the underlying socket. On both HTTP/1 and HTTP/2 the messages are sent to the
+          current process as soon as they arrive, as a firehose. If you wish to maximize request
+          rate or have more control over how messages are streamed, use `into: fun` or
+          `into: collectable` instead.
+
   Response redirect options ([`redirect`](`Req.Steps.redirect/1`) step):
 
     * `:redirect` - if set to `false`, disables automatic response redirects. Defaults to `true`.

lib/req/response_async.ex

@@ -8,7 +8,12 @@ defmodule Req.Response.Async do
   This struct implements the `Enumerale` protocol where each element is a body chunk received
   from the current process mailbox. HTTP Trailer fields are ignored.
 
-  **Note:** this feature is currently experimental and it may change in future releases.
+  If the request is sent using HTTP/11, an extra process is spawned to consume messages from the
+  underlying socket. The messages are sent to the current process as soon as they arrive, as a
+  firehose. If you wish to maximize request rate or have more control over how messages are
+  streamed, use `into: fun` or `into: collectable` instead.
+
+  **Note:** This feature is currently experimental and it may change in future releases.
 
   ## Examples