The compiler's instrumentation pass has some out-of-date comments
about the write barrier and some confusing comments about
typedslicecopy. Update these comments and add a comment to
typedslicecopy explaining why it's manually instrumented while none of
the other operations are.

Change-Id: I024e5361d53f1c3c122db0c85155368a30cabd6b
Reviewed-on: https://go-review.googlesource.com/74430Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>