runtime/debug: clarify SetCrashOutput dup behavior

SetCrashOutput dup's the input file for safety, but I don't think that
the docs are very clear about what the caller can/should do with f. "it
does not close the previous file" is particularly confusing, as it does
close the previous FD (but not the previous passed os.File).

Expand and attempt to clarify the explanation, borrowing wording from
net.FileConn, which also dup's the input os.File.

For #42888.

Change-Id: I1c96d2dce7899e335d8f1cd464d2d9b31aeb4e5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/559800
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/debug/stack.go

@@ -33,9 +33,11 @@ func Stack() []byte {
 
 // SetCrashOutput configures a single additional file where unhandled
 // panics and other fatal errors are printed, in addition to standard error.
-// There is only one additional file: calling SetCrashOutput again
-// overrides any earlier call; it does not close the previous file.
-// SetCrashOutput(nil) disables the use of any additional file.
+// There is only one additional file: calling SetCrashOutput again overrides
+// any earlier call.
+// SetCrashOutput duplicates f's file descriptor, so the caller may safely
+// close f as soon as SetCrashOutput returns.
+// To disable this additional crash output, call SetCrashOutput(nil).
 func SetCrashOutput(f *os.File) error {
 	fd := ^uintptr(0)
 	if f != nil {