man ip-netns: Notice about loose device when do 'del'

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

man/man8/ip-netns.8

@@ -46,6 +46,9 @@ ip-netns \- process network namespace management
 A network namespace is logically another copy of the network stack,
 with its own routes, firewall rules, and network devices.
 
+By default process inherits network namespace from its parent. Initially all
+the processes share the same default network namespace from the init process.
+
 By convention a named network namespace is an object at
 .BR "/var/run/netns/" NAME
 that can be opened.  The file descriptor resulting from opening
@@ -88,14 +91,30 @@ network namespace and assigns NAME.
 .sp
 If NAME is present in /var/run/netns it is umounted and the mount
 point is removed.  If this is the last user of the network namespace the
-network namespace will be freed, otherwise the network namespace
-persists until it has no more users.  ip netns delete may fail if
-the mount point is in use in another mount namespace.
+network namespace will be freed and all physical devices will be moved to the
+default one, otherwise the network namespace persists until it has no more
+users. ip netns delete may fail if the mount point is in use in another mount
+namespace.
 
 If
 .B -all
 option was specified then all the network namespace names will be removed.
 
+It is possible to lose the physical device when it was moved to netns and
+then this netns was deleted with a running process:
+
+    $ ip netns add net0
+    $ ip link set dev eth0 netns net0
+    $ ip netns exec net0 SOME_PROCESS_IN_BACKGROUND
+    $ ip netns del net0
+
+and eth0 will appear in the default netns only after SOME_PROCESS_IN_BACKGROUND
+will exit or will be killed. To prevent this the processes running in net0
+should be killed before deleting the netns:
+
+    $ ip netns pids net0 | xargs kill
+    $ ip netns del net0
+
 .TP
 .B ip netns set NAME NETNSID - assign an id to a peer network namespace
 .sp