Mention required 'flush' in stream_decompress documentation

Author: salty-horse

src/snappy/snappy.py

@@ -298,8 +298,8 @@ def stream_decompress(src,
     support the read method, and 'dst' should support the write method.
 
     The default blocksize is good for almost every scenario.
-    :param decompressor_cls: class that implements `decompress` method like
-        StreamDecompressor in the module
+    :param decompressor_cls: class that implements `decompress` and
+        `flush` methods like StreamDecompressor in the module
     :param start_chunk: start block of data that have already been read from
         the input stream (to detect the format, for example)
     """