path: document that filepath is recommended to manipulate filename paths

Fixes #17690.

Change-Id: Ifd300980aa4c11498ed7c083d08bcdd23f5b307a
Reviewed-on: https://go-review.googlesource.com/32423
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/path/path.go

@@ -4,6 +4,8 @@
 
 // Package path implements utility routines for manipulating slash-separated
 // paths.
+//
+// To manipulate operating system paths, use the path/filepath package.
 package path
 
 import (