cmd/go: clarify and simplify (a little) the description of go test's caching
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.