ptrace(2): Disabling: Describe influence of security.bsd.see_jail_proc

Approved by: re (gjb)
Reviewed by: mhorne, emaste, pauamma_gundo.com
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D41109

(cherry picked from commit d952820105d6a2ad87ddf3bdc6c5fc5215d13b87)
(cherry picked from commit 7c069dbe8e561436b4b2a0d6cf2c2483bd4e0fa3)