Define threading expectation for Stream

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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=72590025

core/src/main/java/com/google/net/stubby/newtransport/Stream.java

@@ -6,6 +6,8 @@ import javax.annotation.Nullable;
 
 /**
  * A single stream of communication between two end-points within a transport.
+ *
+ * <p>An implementation doesn't need to be thread-safe.
  */
 public interface Stream {