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

Fixes: c0d8bab6 ("sctp: add get and set sockopt for reconf_enable")
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>