os/exec: document that examples assume Unix

Fixes #13693.

Change-Id: Ieb89d8a8874ee8c6b304de99dceb07193c26b60e
Reviewed-on: https://go-review.googlesource.com/18315Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec.go

@@ -5,6 +5,10 @@
 // Package exec runs external commands. It wraps os.StartProcess to make it
 // easier to remap stdin and stdout, connect I/O with pipes, and do other
 // adjustments.
+//
+// Note that the examples in this package assume a Unix system.
+// They may not run on Windows, and they do not run in the Go Playground
+// used by golang.org and godoc.org.
 package exec
 
 import (