security/vuxml: Document Emacs arbitrary shell code evaluation

Emacs 29.4 is an emergency bugfix release intended to fix a security
vulnerability. Arbitrary shell commands are no longer run when turning
on Org mode in order to avoid running malicious code.

Reviewed by: ashish, yasu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45702