The old text was written when it was only 1 by default, which
changed a long time ago.

Also add a note that GOMAXPROCS does not limit the total
number of threads.

Change-Id: I104ccd7266d11335320a4d7f5671fb09ed641f88
Reviewed-on: https://go-review.googlesource.com/123916Reviewed-by: Ian Lance Taylor <iant@golang.org>