testing: update docs for Parallel, -cpus flag

[bcmills]

The documentation for testing.T.Parallel (http://golang.org/pkg/testing/#T.Parallel)
says it is "to be run in parallel with (and only with) other parallel tests in this
CPU group".

Nothing on golang.org says what a "CPU group" is or how one determines which
tests are in it.
(https://www.google.com/search?q=";CPU+group"+site%3Agolang.org

[kylelemons]

Comment 1:

When it says "CPU group" it means the group of tests that are running on a particular
number of CPUs.  It's relevant when you specify -test.cpus=1,2,4,8 and such to "go
test."  It looks like the documentation for "go test" and package "testing" omit the
flags, but you can see the flag here:
http://golang.org/src/pkg/testing/testing.go#L109

[rsc]

Comment 2:

Fix:
1. s/ in this CPU group//
2. add -cpus flag to go help testflag and package testing docs.

Labels changed: added priority-later, removed priority-triage.

Status changed to Accepted.

[rsc]

Comment 3:

Labels changed: added size-s.

[rsc]

Comment 4:

Labels changed: added suggested.

[rsc]

Comment 5:

This issue was closed by revision 89a2a8c.

Status changed to Fixed.