txhash: Add txrehash sysctl description

Update Documentation/admin-guide/sysctl/net.rst with txrehash usage
description.
Signed-off-by: Akhmat Karakotov <hmukos@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/admin-guide/sysctl/net.rst

@@ -365,6 +365,15 @@ new netns has been created.
 
 Default : 0  (for compatibility reasons)
 
+txrehash
+--------
+
+Controls default hash rethink behaviour on listening socket when SO_TXREHASH
+option is set to SOCK_TXREHASH_DEFAULT (i. e. not overridden by setsockopt).
+
+If set to 1 (default), hash rethink is performed on listening socket.
+If set to 0, hash rethink is not performed.
+
 2. /proc/sys/net/unix - Parameters for Unix domain sockets
 ----------------------------------------------------------