exec: add a little package commentary explaining its

relationship to os.Process.

R=golang-dev, rsc1
CC=golang-dev
https://golang.org/cl/4384041

src/pkg/exec/exec.go

@@ -2,9 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// The exec package runs external commands.
+// The exec package 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.
 package exec
 
+// BUG(r): This package should be made even easier to use or merged into os.
+
 import (
 	"os"
 	"strconv"