encoding/hex: implement examples using all exported functions

encoding/hex: implement examples using all exported functions

Fixes #11254.
Updates #16360.

Implements examples using all exported functions.

This CL also updates Decode documentation to
state that only hexadecimal characters are accepted
in the source slice src, but also that the length
of src must be even.

Change-Id: Id016a4ba814f940cd300f26581fb4b9d2aded306
Reviewed-on: https://go-review.googlesource.com/28482
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>