path, path/filepath: clarify and cross-reference packages

The path package has a reference to the path/filepath package, so add
a reverse reference.

And clarify the path package doesn't do Windows paths.

Fixes #20117

Change-Id: I65c5ce24e600b32ea20c5821b744bd89f6aff98c
Reviewed-on: https://go-review.googlesource.com/45653Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/path/filepath/path.go

@@ -4,6 +4,11 @@
 
 // Package filepath implements utility routines for manipulating filename paths
 // in a way compatible with the target operating system-defined file paths.
+//
+// The filepath package uses either forward slashes or backslashes,
+// depending on the operating system. To process paths such as URLs
+// that always use forward slashes regardless of the operating
+// system, see the path package.
 package filepath
 
 import (

src/path/path.go

@@ -5,7 +5,10 @@
 // Package path implements utility routines for manipulating slash-separated
 // paths.
 //
-// To manipulate operating system paths, use the path/filepath package.
+// The path package should only be used for paths separated by forward
+// slashes, such as the paths in URLs. This package does not deal with
+// Windows paths with drive letters or backslashes; to manipulate
+// operating system paths, use the path/filepath package.
 package path
 
 import (