path: document that Clean deletes trailing slashes.

Fixes #3492.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6112044

src/pkg/path/filepath/path.go

@@ -30,6 +30,9 @@ const (
 //	   that is, replace "/.." by "/" at the beginning of a path,
 //         assuming Separator is '/'.
 //
+// The returned path ends in a slash only if it represents a root directory,
+// such as "/" on Unix or `C:\` on Windows.
+//
 // If the result of this process is an empty string, Clean
 // returns the string ".".
 //

src/pkg/path/path.go

@@ -21,6 +21,8 @@ import (
 //	4. Eliminate .. elements that begin a rooted path:
 //	   that is, replace "/.." by "/" at the beginning of a path.
 //
+// The returned path ends in a slash only if it is the root "/".
+//
 // If the result of this process is an empty string, Clean
 // returns the string ".".
 //