Documentation: add description for net.sctp.ecn_enable

Describe it in networking/ip-sysctl.rst like other SCTP options.

Fixes: 2f5268a9 ("sctp: allow users to set netns ecn flag with sysctl")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>