Documentation: add description for net.sctp.reconf_enable

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: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>