cmd/go: document that BinaryOnly packages must have accurate import info

Update BinaryOnly test by adding import _ "fmt".

Change-Id: I3a1dcfb83a27d8ff50a658060a46e1a3f481f6c7
Reviewed-on: https://go-review.googlesource.com/56276Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -917,8 +917,10 @@
 // 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. See the go/build package documentation
-// for more details.
+// 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.
 //
 //
 // GOPATH environment variable

src/cmd/go/go_test.go

@@ -3618,6 +3618,7 @@ func TestBinaryOnlyPackages(t *testing.T) {
 	tg.tempFile("src/p1/missing.go", `//go:binary-only-package
 
 		package p1
+		import _ "fmt"
 		func G()
 	`)
 	tg.wantNotStale("p1", "no source code", "should NOT want to rebuild p1 (first)")

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

@@ -565,8 +565,10 @@ 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. See the go/build package documentation
-for more details.
+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.
 	`,
 }