path/filepath: add examples for Base, Dir and IsAbs
path/filepath: add examples for Base, Dir and IsAbs

Change-Id: I7a438409748f0f9d6517a7ea1cdee6512ce3ca8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/161678
Run-TryBot: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

josharian/go,CAFxX/go,josharian/go,cloudflare/go,cloudflare/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,golang/go,akutz/go,akutz/go,akutz/go,josharian/go,nawawi/go,nawawi/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,josharian/go,akutz/go,josharian/go,akutz/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,josharian/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,golang/go,cloudflare/go,golang/go,nawawi/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,CAFxX/go