runtime: update docs for GOMAXPROCS

https://github.com/golang/go/blob/master/doc/effective_go.html#L3211 is used to update the docs comment for `GOMAXPROCS` function.

Fixes #41275

Change-Id: I39f58e93a267c6e9f3ac6638ed51acbe5284ada2
GitHub-Last-Rev: e45c8ac
GitHub-Pull-Request: #41276
Reviewed-on: https://go-review.googlesource.com/c/go/+/253537
Reviewed-by: Keith Randall <[email protected]>

Author: ipriver

src/runtime/debug.go

@@ -10,9 +10,8 @@ import (
 )
 
 // GOMAXPROCS sets the maximum number of CPUs that can be executing
-// simultaneously and returns the previous setting. If n < 1, it does not
-// change the current setting.
-// The number of logical CPUs on the local machine can be queried with NumCPU.
+// simultaneously and returns the previous setting. It defaults to
+// the value of runtime.NumCPU. If n < 1, it does not change the current setting.
 // This call will go away when the scheduler improves.
 func GOMAXPROCS(n int) int {
 	if GOARCH == "wasm" && n > 1 {