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: 4371886
GitHub-Pull-Request: #53647
Reviewed-on: https://go-review.googlesource.com/c/go/+/415774
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: cristiangreco

src/go/build/constraint/expr.go

@@ -5,9 +5,7 @@
 // Package constraint implements parsing and evaluation of build constraint lines.
 // See https://golang.org/cmd/go/#hdr-Build_constraints for documentation about build constraints themselves.
 //
-// This package parses both the original “// +build” syntax and the “//go:build” syntax that will be added in Go 1.17.
-// The parser is being included in Go 1.16 to allow tools that need to process Go 1.17 source code
-// to still be built against the Go 1.16 release.
+// This package parses both the original “// +build” syntax and the “//go:build” syntax that was added in Go 1.17.
 // See https://golang.org/design/draft-gobuild for details about the “//go:build” syntax.
 package constraint