Redis: update configuration bindmount example.

The provided example may be misleading and will not work in certain
scenarios, where Redis attempts to create additional configuration files
or rewrite existing ones.

See redis/redis#8172.

redis/content.md

@@ -51,11 +51,13 @@ CMD [ "redis-server", "/usr/local/etc/redis/redis.conf" ]
 Alternatively, you can specify something along the same lines with `docker run` options.
 
 ```console
-$ docker run -v /myredis/conf/redis.conf:/usr/local/etc/redis/redis.conf --name myredis %%IMAGE%% redis-server /usr/local/etc/redis/redis.conf
+$ docker run -v /myredis/conf:/usr/local/etc/redis --name myredis %%IMAGE%% redis-server /usr/local/etc/redis/redis.conf
 ```
 
 Where `/myredis/conf/` is a local directory containing your `redis.conf` file. Using this method means that there is no need for you to have a Dockerfile for your redis container.
 
+The mapped directory should be writable, as depending on the configuration and mode of operation, Redis may need to create additional configuration files or rewrite existing ones.
+
 ## `32bit` variant
 
 This variant is *not* a 32bit image (and will not run on 32bit hardware), but includes Redis compiled as a 32bit binary, especially for users who need the decreased memory requirements associated with that. See ["Using 32 bit instances"](http://redis.io/topics/memory-optimization#using-32-bit-instances) in the Redis documentation for more information.