libc: Note that getentropy is nearly POSIX 2024

Our implementation currently diverges from POSIX 2024 in a couple of
ways, as now noted in the BUGS section.

Reviewed by: brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47589

(cherry picked from commit 95b71a659a9bdc6e9071d80c7369a935c2bc16f4)