ports(7): '*-list' targets: Add/fix doc on whether they are recursive

Signed-off-by: Olivier Certner <olce.freebsd@certner.fr>
Reviewed-by: imp
Pull-request: https://github.com/freebsd/freebsd-src/pull/691

(cherry picked from commit d0b0bc37a3a5655c4979fbed206cfb6a30210ac0)