runtime: clarify that NumCPU returns only available CPUs

Update #11609.

Change-Id: Ie363facf13f5e62f1af4a8bdc42a18fb36e16ebf
Reviewed-on: https://go-review.googlesource.com/12022Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/debug.go

@@ -31,7 +31,7 @@ func GOMAXPROCS(n int) int {
 	return ret
 }
 
-// NumCPU returns the number of logical CPUs on the local machine.
+// NumCPU returns the number of logical CPUs usable by the current process.
 func NumCPU() int {
 	return int(ncpu)
 }