quota: Update documentation

Mention that quota netlink messages are sent only in initial network
namespace.
Signed-off-by: Jan Kara <jack@suse.cz>

Documentation/filesystems/quota.txt

@@ -32,7 +32,10 @@ The interface uses generic netlink framework (see
 http://lwn.net/Articles/208755/ and http://people.suug.ch/~tgr/libnl/ for more
 details about this layer). The name of the quota generic netlink interface
 is "VFS_DQUOT". Definitions of constants below are in <linux/quota.h>.
-  Currently, the interface supports only one message type QUOTA_NL_C_WARNING.
+Since the quota netlink protocol is not namespace aware, quota netlink messages
+are sent only in initial network namespace.
+
+Currently, the interface supports only one message type QUOTA_NL_C_WARNING.
 This command is used to send a notification about any of the above mentioned
 events. Each message has six attributes. These are (type of the argument is
 in parentheses):