doc/go1.16: improve channel race detector changes description

Based on text from Daniel Fava.

For #40700.

Change-Id: I0bc3a4340b8a777ff96d3cf226a7d51d3f65db2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/275786


Trust: Austin Clements <austin@google.com>
Reviewed-by: Daniel Fava <danielsfava@gmail.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>