encoding/asn1: document what Unmarshal returns in rest

Specifically, this change documents the behavior of Unmarshal when a
SEQUENCE contains trailing elements.

For context Unmarshal treats trailing elements of a SEQUENCE that do not
have matching struct fields as valid, as this is how ASN.1 structures
are typically extended. This can be somewhat confusing as you might
expect those elements to be appended to rest, but rest is really only
for trailing data unrelated to the structure being parsed (i.e. if you
append a second sequence to b, it would be returned in rest).

Fixes #35680

Change-Id: Ia2c68b2f7d8674d09e859b4b7f9aff327da26fa0
Reviewed-on: https://go-review.googlesource.com/c/go/+/233537


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>