os: clarify that ModePerm permission bits are the Unix ones

Fixes #7075

Change-Id: I57fcebaaef768b00263b6b91211b1434f6a4b229
Reviewed-on: https://go-review.googlesource.com/8919Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/os/types.go

@@ -53,7 +53,7 @@ const (
 	// Mask for the type bits. For regular files, none will be set.
 	ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice
 
-	ModePerm FileMode = 0777 // permission bits
+	ModePerm FileMode = 0777 // Unix permission bits
 )
 
 func (m FileMode) String() string {