net/http: add comment to clarify whether Dir is '/' or '\'.

LGTM=bradfitz
R=bradfitz, alex.brainman
CC=golang-codereviews
https://golang.org/cl/168600044

src/net/http/fs.go

@@ -22,8 +22,12 @@ import (
 	"time"
 )
 
-// A Dir implements http.FileSystem using the native file
-// system restricted to a specific directory tree.
+// A Dir implements FileSystem using the native file system restricted to a
+// specific directory tree.
+//
+// While the FileSystem.Open method takes '/'-separated paths, a Dir's string
+// value is a filename on the native file system, not a URL, so it is separated
+// by filepath.Separator, which isn't necessarily '/'.
 //
 // An empty Dir is treated as ".".
 type Dir string