io: clarify that Read(nil) can return 0, EOF

io: clarify that Read(nil) can return 0, EOF

Fixes #40385

Change-Id: I965b5db985fd4418a992e883073cbc8309b2cb88
Reviewed-on: https://go-review.googlesource.com/c/go/+/498355
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>