runtime: make it clear that Goexit cannot be recover'd.

LGTM=r
R=r, bradfitz, khr
CC=golang-codereviews
https://golang.org/cl/136660044

src/runtime/panic.go

@@ -238,7 +238,8 @@ func deferreturn(arg0 uintptr) {
 }
 
 // Goexit terminates the goroutine that calls it.  No other goroutine is affected.
-// Goexit runs all deferred calls before terminating the goroutine.
+// Goexit runs all deferred calls before terminating the goroutine.  Because Goexit
+// is not panic, however, any recover calls in those deferred functions will return nil.
 //
 // Calling Goexit from the main goroutine terminates that goroutine
 // without func main returning. Since func main has not returned,