os: document FileInfo.Size as system-dependent for irregular files

R=rsc, r
CC=golang-dev
https://golang.org/cl/5795059

src/pkg/os/types.go

@@ -15,7 +15,7 @@ func Getpagesize() int { return syscall.Getpagesize() }
 // A FileInfo describes a file and is returned by Stat and Lstat
 type FileInfo interface {
 	Name() string       // base name of the file
-	Size() int64        // length in bytes
+	Size() int64        // length in bytes for regular files; system-dependent for others
 	Mode() FileMode     // file mode bits
 	ModTime() time.Time // modification time
 	IsDir() bool        // abbreviation for Mode().IsDir()