Update docs/current/ to mention sysupgrade

[coypu%SDF.ORG@localhost]

Hello.

I believe nearly the entire segment (too much to list) of the following link:
http://www.netbsd.org/docs/current/#updating-from-snapshot

can be replaced by:
"Obtain the sysutils/sysupgrade package and run:
sysupgrade auto http://nyftp.netbsd.org/pub/NetBSD-daily/HEAD/TIME/ARCH/";

Perhaps remove starting from the first step.

This will be much easier to follow and less intimidating for beginners.
Thank you.