Document that all errors from RPCs are status errors (grpc#1782)

call.go

@@ -138,6 +138,8 @@ func sendRequest(ctx context.Context, dopts dialOptions, compressor Compressor,
 
 // Invoke sends the RPC request on the wire and returns after response is
 // received.  This is typically called by generated code.
+//
+// All errors returned by Invoke are compatible with the status package.
 func (cc *ClientConn) Invoke(ctx context.Context, method string, args, reply interface{}, opts ...CallOption) error {
 	if cc.dopts.unaryInt != nil {
 		return cc.dopts.unaryInt(ctx, method, args, reply, cc, invoke, opts...)

stream.go

@@ -50,6 +50,8 @@ type StreamDesc struct {
 }
 
 // Stream defines the common interface a client or server stream has to satisfy.
+//
+// All errors returned from Stream are compatible with the status package.
 type Stream interface {
 	// Context returns the context for this stream.
 	Context() context.Context