os: document that Exit does not run deferred calls

Fixes #4101.

R=dsymonds, rsc
CC=golang-dev
https://golang.org/cl/7065048

src/pkg/os/proc.go

@@ -31,4 +31,6 @@ func Getgroups() ([]int, error) {
 
 // Exit causes the current program to exit with the given status code.
 // Conventionally, code zero indicates success, non-zero an error.
+// The program terminates immediately; deferred functions are
+// not run.
 func Exit(code int) { syscall.Exit(code) }