Make it clear that IsValid checks that we have valid
reflect.Value and not the value of `v`

fixes #34152

Change-Id: Ib3d359eeb3a82bf733b9ed17c777fc4c143bc29c
Reviewed-on: https://go-review.googlesource.com/c/go/+/193841Reviewed-by: Ian Lance Taylor <iant@golang.org>