doc: allow buffered channel as semaphore without initialization

doc: allow buffered channel as semaphore without initialization

This rule not existing has been the source of many discussions
on golang-dev and on issues. We have stated publicly that it is
true, but we have never written it down. Write it down.

Fixes #6242.

LGTM=r, dan.kortschak, iant, dvyukov
R=golang-codereviews, r, dominik.honnef, dvyukov, dan.kortschak, iant, 0xjnml
CC=golang-codereviews
https://golang.org/cl/75130045