Edits to plugin docs after building authz plugin

Signed-off-by: Riyaz Faizullabhoy <riyaz.faizullabhoy@docker.com>

docs/extend/config.md

@@ -55,6 +55,8 @@ Config provides the base accessible fields for working with V0 plugin format
 
       	- **docker.volumedriver/1.0**
 
+      	- **docker.authz/1.0**
+
     - **`socket`** *string*
 
       socket is the name of the socket the engine should use to communicate with the plugins.

docs/extend/index.md

@@ -199,8 +199,8 @@ drwx------ 19 root root 4096 Aug  8 17:56 rootfs
 The `rootfs` directory represents the root filesystem of the plugin. In this
 example, it was created from a Dockerfile:
 
->**Note:** The `/run/docker/plugins` directory is mandatory for docker to communicate with
-the plugin.
+>**Note:** The `/run/docker/plugins` directory is mandatory inside of the
+plugin's filesystem for docker to communicate with the plugin.
 
 ```bash
 $ git clone https://github.com/vieux/docker-volume-sshfs