runtime: document that NumCPU does not change

Fixes #11609.

Change-Id: I3cf64164fde28ebf739706728b84d8ef5b6dc90e
Reviewed-on: https://go-review.googlesource.com/17456
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/debug.go

@@ -35,6 +35,10 @@ func GOMAXPROCS(n int) int {
 }
 
 // NumCPU returns the number of logical CPUs usable by the current process.
+//
+// The set of available CPUs is checked by querying the operating system
+// at process startup. Changes to operating system CPU allocation after
+// process startup are not reflected.
 func NumCPU() int {
 	return int(ncpu)
 }