Update obsolete comment about pg_usleep() accuracy.

Update obsolete comment about pg_usleep() accuracy.

There are still some systems that use traditional tick-based sleep
timing, but many including Linux, FreeBSD and macOS started using high
resolution timer hardware more directly a decade or two ago.  Update our
comment about that.  Also highlight that Windows is like the older
Unixen in that respect.

Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BogAon8_V223Ldv6taPR2uKH3X_UJ_A7LJAf3-VRARPA%40mail.gmail.com