build/constraint: update doc to mention a feature added in Go 1.17
build/constraint: update doc to mention a feature added in Go 1.17

The pkg documentation mentions that the "//go:build" syntax "will be"
added in Go 1.17. In fact, it has been added in that Go release, so the
documentation can now be updated.

Change-Id: I72f24063c3be62d97ca78bf724d56599f5f19460
GitHub-Last-Rev: 4371886f6ce9f2c2a370df047a5baa1f122c681f
GitHub-Pull-Request: golang/go#53647
Reviewed-on: https://go-review.googlesource.com/c/go/+/415774
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Benny Siegert <05f4aee36fd25d0de2a07789fd6e73398db492cc@gmail.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,golang/go,golang/go,golang/go,nawawi/go