Rework seekable body documentation with the split into two plugins

Author: joelwurtz

plugins/always-seekable-body-plugin.rst

@@ -10,7 +10,7 @@ request or you can even start a completely new request. This gives you full cont
 
    introduction
    build-your-own
-   always-seekable-body-plugin
+   seekable-body-plugins
    authentication
    cache
    content-length

plugins/index.rst

@@ -0,0 +1,30 @@
+Seekable Body Plugins
+=====================
+
+``RequestSeekableBodyPlugin`` and ``ResponseSeekableBodyPlugin`` ensure that body used in request and response is always seekable.
+This allows a lot of components, reading the stream, to rewind it in order to be used later by another component::
+
+    use Http\Discovery\HttpClientDiscovery;
+    use Http\Client\Common\PluginClient;
+    use Http\Client\Common\Plugin\RequestSeekableBodyPlugin;
+    use Http\Client\Common\Plugin\ResponseSeekableBodyPlugin;
+
+    $options = [
+        'use_file_buffer' => true,
+        'memory_buffer_size' => 2097152,
+    ];
+    $requestSeekableBodyPlugin = new RequestSeekableBodyPlugin($options);
+    $responseSeekableBodyPlugin = new ResponseSeekableBodyPlugin($options);
+
+    $pluginClient = new PluginClient(
+        HttpClientDiscovery::find(),
+        [$requestSeekableBodyPlugin, $responseSeekableBodyPlugin]
+    );
+
+Those plugins support the following options (which are passed to the ``BufferedStream`` class):
+
+ * ``use_file_buffer``: Whether it should use a temporary file to buffer the body of a stream if it's too big
+ * ``memory_buffer_size``: Maximum memory to use for buffering the stream before it switch to a file
+
+``RequestSeekableBodyPlugin`` should be added in top of your plugins, then next plugins can seek request body (i.e. for logging purpose).
+``ResponseSeekableBodyPlugin`` should be the last plugin, then previous plugins can seek response body.