Ads documentation for both formats of messages accepted by
ReadResponse(). Validity of message should not be altered by
the validation process. On message with unexpected code,
a properly formatted message was not fully read.

Fixes #10230

Change-Id: Ic0b473059a68ab624ce0525e359d0f5d0b8d2117
Reviewed-on: https://go-review.googlesource.com/18172Reviewed-by: Russ Cox <rsc@golang.org>