sync.8: Document that the "sync dance" is not a thing

People still believe that it is essential to run sync(8) a couple of
times before a reboot/halt. Document that this has not been necessary
for a long time now.

Reviewed by: imp, bcr, Pau Amma <pauamma@gundo.com>
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D33233

(cherry picked from commit 177ba183076bd6fe6b29b1d657b954cca256506b)