MAC: 'kernel_mac_support' module: Make an outdated comment more generic

No functional change.

Reviewed by: jamie
Approved by: markj (mentor)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46898

(cherry picked from commit 90678c892d7b3a90339b7fc19fde16c64fe3cb70)