Add docs for `docker network create --attachable`

Signed-off-by: Harald Albers <github@albersweb.de>
(cherry picked from commit 7433d3acf6)
Signed-off-by: Victor Vieux <vieux@docker.com>

docs/reference/commandline/network_create.md

@@ -21,6 +21,7 @@ Usage:	docker network create [OPTIONS] NETWORK
 Create a network
 
 Options:
+      --attachable           Enable manual container attachment
       --aux-address value    Auxiliary IPv4 or IPv6 addresses used by Network
                              driver (default map[])
   -d, --driver string        Driver to manage the Network (default "bridge")

man/docker-network-create.1.md

@@ -6,6 +6,7 @@ docker-network-create - create a new network
 
 # SYNOPSIS
 **docker network create**
+[**--attachable**]
 [**--aux-address**=*map[]*]
 [**-d**|**--driver**=*DRIVER*]
 [**--gateway**=*[]*]
@@ -143,6 +144,9 @@ to create an externally isolated `overlay` network, you can specify the
 `--internal` option.
 
 # OPTIONS
+**--attachable**
+  Enable manual container attachment
+
 **--aux-address**=map[]
   Auxiliary IPv4 or IPv6 addresses used by network driver