net: document methods of Buffers

There is code in the wild that copies the Buffers slice,
but not the contents.
Let's document explicitly that it is not safe to do so.

Updates #45163

Change-Id: Id45e27b93037d4e9f2bfde2558e7869983b60bcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/387434


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>