runtime/protoiface: update package documentation

Contrary to the WARNING, this package should be stable.
However, it still should not be imported by the end-user.

Change-Id: I05b4b9ebb1e0d28ab626f8c51fbe827b5acf237e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/178545
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/runtime/protoiface/legacy.go b/runtime/protoiface/legacy.go
index c560ba8..56f3e78 100644
--- a/runtime/protoiface/legacy.go
+++ b/runtime/protoiface/legacy.go
@@ -2,11 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package protoiface contains types referenced by generated messages.
-//
-// WARNING: This package should only ever be imported by generated messages.
-// The compatibility agreement covers nothing except for functionality needed
-// to keep existing generated messages operational.
 package protoiface
 
 import (
diff --git a/runtime/protoiface/methods.go b/runtime/protoiface/methods.go
index af86d62..e4a5157 100644
--- a/runtime/protoiface/methods.go
+++ b/runtime/protoiface/methods.go
@@ -2,6 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// Package protoiface contains types referenced or implemented by messages.
+//
+// WARNING: This package should only be imported by message implementations.
+// The functionality found in this package should be accessed through
+// higher-level abstractions provided by the proto package.
 package protoiface
 
 import (