Specify Status*Exception is to be used in StreamObserver.onError

core/src/main/java/io/grpc/Status.java

@@ -392,6 +392,8 @@ public final class Status {
 
   /**
    * Extract an error {@link Status} from the causal chain of a {@link Throwable}.
+   *
+   * @return non-{@code null} status
    */
   public static Status fromThrowable(Throwable t) {
     for (Throwable cause : Throwables.getCausalChain(t)) {

stub/src/main/java/io/grpc/stub/StreamObserver.java

@@ -68,6 +68,12 @@ public interface StreamObserver<V>  {
    * exception is thrown by an implementation of {@code onError} no further calls to any method are
    * allowed.
    *
+   * <p>{@code t} should be a {@link io.grpc.StatusException} or {@link
+   * io.grpc.StatusRuntimeException}, but other {@code Throwable} types are possible. Callers should
+   * generally convert from a {@link io.grpc.Status} via {@link io.grpc.Status#asException()} or
+   * {@link io.grpc.Status#asRuntimeException()}. Implementations should generally convert to a
+   * {@code Status} via {@link io.grpc.Status#fromThrowable(Throwable)}.
+   *
    * @param t the error occurred on the stream
    */
   void onError(Throwable t);