Update doc to specify that users should request the fields needed in the
Package struct by using the NeedX bits directly.
Change-Id: I9213827fa7e5d01800d79173fe5161f39ffda85e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/173959
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>