runtime: clarify finalizer semantics for tiny objects

runtime: clarify finalizer semantics for tiny objects

This change clarifies that a finalizer is not guaranteed to run,
not only for zero bytes objects but also tiny objects (< 16bytes).

Fixes #46827

Change-Id: I193e77f6f024c79110604f86bcb1a28b16cf98ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/337391
Run-TryBot: Changkun Ou <mail@changkun.de>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>