media: clarify that flush() is not supported for encoders.

 most of its life. When you queue an input buffer with the {@linkplain #BUFFER_FLAG_END_OF_STREAM

 end-of-stream marker}, the codec transitions to the End-of-Stream sub-state. In this state the

 codec no longer accepts further input buffers, but still generates output buffers until the

 end-of-stream is reached on the output. You can move back to the Flushed sub-state at any time

 while in the Executing state using {@link #flush}.

 end-of-stream is reached on the output. For decoders, you can move back to the Flushed sub-state

 at any time while in the Executing state using {@link #flush}.

 <p class=note>

 <strong>Note:</strong> Going back to Flushed state is only supported for decoders, and may not

 work for encoders (the behavior is undefined).

 <p>

 Call {@link #stop} to return the codec to the Uninitialized state, whereupon it may be configured

 again. When you are done using a codec, you must release it by calling {@link #release}.