Add a comment so people don't accidentally new StreamableHashMap(), and expect it to work.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@6106 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/as/com/threerings/util/StreamableHashMap.as

@@ -40,6 +40,14 @@ import com.threerings.util.maps.ImmutableMap;
 public class StreamableHashMap extends ForwardingMap
     implements Streamable
 {
+    /**
+     * Creates a new StreamableHashMap.
+     *
+     * @param keyClazz The class to use as the map key. May be null, but this functionality is
+     * provided only for deserialization. A StreamableHashMap that has not been initialized
+     * properly by either a non-null keyClazz or via readObject will throw errors on write and
+     * always return null on read.
+     */
     public function StreamableHashMap (keyClazz :Class = null)
     {
         super(keyClazz == null ? DEFAULT_MAP : Maps.newMapOf(keyClazz));