Add a slightly expanded version of the Decoder type comment to the top
level package doc, which explains that this package is not designed
to be hardened against adversarial inputs.
Change-Id: I8b83433838c8235eb06ded99041fdf726c811ee5
Reviewed-on: https://go-review.googlesource.com/c/go/+/436096
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org> Reviewed-by: Damien Neil <dneil@google.com>