path/filepath: document Clean behavior for each function

Document explicitly which functions Clean the result rather than
documenting it in the package comment.

Updates #10122.
Fixes #16111.

Change-Id: Ia589c7ee3936c9a6a758725ac7f143054d53e41e
Reviewed-on: https://go-review.googlesource.com/24747


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>