rights.4: add note about rights not being simple bitmasks

PR: 277057
Reviewed by: oshogbo, asomers
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44473

(cherry picked from commit 537bdafbc25d39b4bb3dce124691913f86c08288)
(cherry picked from commit 6ac10e8a72a730e9078738e317f5c3e2475bd7bd)