doc: release: 4.2.0: add sys_clock gettime settime nanosleep notes

Add bullets to release notes for sys_clock_gettime(),
sys_clock_settime(), and sys_clock_nanosleep().

These changes were significant, as they make timekeeping dependencies
more independent between libc and posix, and remove several unnecessary
dependencies on posix for ISO C library routines.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>