We're still leaving open the possibility of changing this API,
but things have baked for a bit so I feel comfortable removing the
build tag.
Also add some documentation.
Change-Id: I3beb666b58177553fc406dc9670d569d5928fedd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/189460
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>