encoding/gob: add top level security doc
encoding/gob: add top level security doc

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 <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
Auto-Submit: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go