doc: rearrange the description of GOMAXPROCS
doc: rearrange the description of GOMAXPROCS

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/123916
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,akutz/go,CAFxX/go,golang/go,golang/go,cloudflare/go,golang/go,cloudflare/go,josharian/go,golang/go,josharian/go,golang/go,CAFxX/go,golang/go,cloudflare/go,josharian/go,nawawi/go,akutz/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,nawawi/go,cloudflare/go,akutz/go,josharian/go,akutz/go,josharian/go,CAFxX/go,akutz/go,akutz/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,josharian/go,nawawi/go,akutz/go