media: document MediaFormat.KEY_FRAME_RATE better

 * <tr><td>{@link #KEY_HEIGHT}</td><td>Integer</td><td></td></tr>

 * <tr><td>{@link #KEY_COLOR_FORMAT}</td><td>Integer</td><td>set by the user

 *         for encoders, readable in the output format of decoders</b></td></tr>

 * <tr><td>{@link #KEY_FRAME_RATE}</td><td>Integer or Float</td><td><b>encoder-only</b></td></tr>

 * <tr><td>{@link #KEY_FRAME_RATE}</td><td>Integer or Float</td><td>required for <b>encoders</b>,

 *         optional for <b>decoders</b></td></tr>

 * <tr><td>{@link #KEY_CAPTURE_RATE}</td><td>Integer</td><td></td></tr>

 * <tr><td>{@link #KEY_I_FRAME_INTERVAL}</td><td>Integer</td><td><b>encoder-only</b></td></tr>

 * <tr><td>{@link #KEY_INTRA_REFRESH_PERIOD}</td><td>Integer</td><td><b>encoder-only</b>, optional</td></tr>

    /**

     * A key describing the frame rate of a video format in frames/sec.

     * The associated value is an integer or a float.

     * The associated value is normally an integer when the value is used by the platform,

     * but video codecs also accept float configuration values.

     * Specifically, {@link MediaExtractor#getTrackFormat MediaExtractor} provides an integer

     * value corresponding to the frame rate information of the track if specified and non-zero.

     * Otherwise, this key is not present. {@link MediaCodec#configure MediaCodec} accepts both

     * float and integer values. This represents the desired operating frame rate if the

     * {@link #KEY_OPERATING_RATE} is not present and {@link #KEY_PRIORITY} is {@code 0}

     * (realtime). For video encoders this value corresponds to the intended frame rate,

     * although encoders are expected

     * to support variable frame rate based on {@link MediaCodec.BufferInfo#presentationTimeUs

     * buffer timestamp}. This key is not used in the {@code MediaCodec}

     * {@link MediaCodec#getInputFormat input}/{@link MediaCodec#getOutputFormat output} formats,

     * nor by {@link MediaMuxer#addTrack MediaMuxer}.

     */

    public static final String KEY_FRAME_RATE = "frame-rate";