Describe it in networking/ip-sysctl.rst like other SCTP options.
We need to document this especially as when using the feature
of User Message Interleaving, some socket options also needs
to be set.

Fixes: 463118c3 ("sctp: support sysctl to allow users to use stream interleave")
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Acked-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>