I found the previous text choppy and hard to follow, and in putting
this CL together, based entirely on the existing text, I found
several details that seemed misleading to me.

This is my attempt to make the text simultaneously easier to
understand, more complete, and more precise. I may have failed in
all three, but I wanted to try.

Change-Id: I088cb457f6fcad8f2b40236949cc3ac43455e600
Reviewed-on: https://go-review.googlesource.com/87735Reviewed-by: Russ Cox <rsc@golang.org>