cmd/go: mention that binary packages are going away

Updates #28152
Fixes #29927

Change-Id: Iea692c90074d057a1733e98bca3928e8f3569585
Reviewed-on: https://go-review.googlesource.com/c/159557
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -1598,14 +1598,14 @@
 // line comment. See the go/build package documentation for
 // more details.
 //
-// Non-test Go source files can also include a //go:binary-only-package
-// comment, indicating that the package sources are included
-// for documentation only and must not be used to build the
-// package binary. This enables distribution of Go packages in
-// their compiled form alone. Even binary-only packages require
-// accurate import blocks listing required dependencies, so that
-// those dependencies can be supplied when linking the resulting
-// command.
+// Through the Go 1.12 release, non-test Go source files can also include
+// a //go:binary-only-package comment, indicating that the package
+// sources are included for documentation only and must not be used to
+// build the package binary. This enables distribution of Go packages in
+// their compiled form alone. Even binary-only packages require accurate
+// import blocks listing required dependencies, so that those
+// dependencies can be supplied when linking the resulting command.
+// Note that this feature is scheduled to be removed after the Go 1.12 release.
 //
 //
 // The go.mod file

src/cmd/go/internal/help/helpdoc.go

@@ -636,14 +636,14 @@ at the first item in the file that is not a blank line or //-style
 line comment. See the go/build package documentation for
 more details.
 
-Non-test Go source files can also include a //go:binary-only-package
-comment, indicating that the package sources are included
-for documentation only and must not be used to build the
-package binary. This enables distribution of Go packages in
-their compiled form alone. Even binary-only packages require
-accurate import blocks listing required dependencies, so that
-those dependencies can be supplied when linking the resulting
-command.
+Through the Go 1.12 release, non-test Go source files can also include
+a //go:binary-only-package comment, indicating that the package
+sources are included for documentation only and must not be used to
+build the package binary. This enables distribution of Go packages in
+their compiled form alone. Even binary-only packages require accurate
+import blocks listing required dependencies, so that those
+dependencies can be supplied when linking the resulting command.
+Note that this feature is scheduled to be removed after the Go 1.12 release.
 	`,
 }