Clarify that Compressor and Decompressor callbacks must support being invoked
concurrently, but that the writer or reader returned need not be.

Updates #8359

Change-Id: Ia407b581dd124185f165c25f5701018a8ce4357a
Reviewed-on: https://go-review.googlesource.com/18627Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>