doc/go1.17: document //go:build lines

In 1.17, //go:build lines are fully supported. This entails changes to
the go command, vet, and gofmt. Document all of them.

I'm not Russ, but this is a significant change, it slipped under the
radar, and we're trying to get the release out. So here's what I got.

I wasn't sure where to put the go command change. On the one hand,
it's pretty significant. On the other, it certainly affects fewer
people than lazy loading. So it probably shouldn't be first, but I also
didn't want to bury it the middle of all the other module changes. Open
to suggestions.

Change-Id: Ia1a96bcfb1977973c5b0b0a6b18a9242a745af12
Reviewed-on: https://go-review.googlesource.com/c/go/+/326209


Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>