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

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/45653
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>