Make it clear that non-optional MediaFormat keys are required

 *

 * The format of the media data is specified as string/value pairs.

 *

 * Keys common to all formats:

 * Keys common to all formats, <b>all keys not marked optional are mandatory</b>:

 *

 * <table>

 * <tr><th>Name</th><th>Value Type</th><th>Description</th></tr>

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

 * <tr><td>{@link #KEY_IS_ADTS}</td><td>Integer</td><td>optional, if <em>decoding</em> AAC audio content, setting this key to 1 indicates that each audio frame is prefixed by the ADTS header.</td></tr>

 * <tr><td>{@link #KEY_AAC_PROFILE}</td><td>Integer</td><td><b>encoder-only</b>, optional, if content is AAC audio, specifies the desired profile.</td></tr>

 * <tr><td>{@link #KEY_CHANNEL_MASK}</td><td>Integer</td><td>A mask of audio channel assignments</td></tr>

 * <tr><td>{@link #KEY_CHANNEL_MASK}</td><td>Integer</td><td>optional, a mask of audio channel assignments</td></tr>

 * <tr><td>{@link #KEY_FLAC_COMPRESSION_LEVEL}</td><td>Integer</td><td><b>encoder-only</b>, optional, if content is FLAC audio, specifies the desired compression level.</td></tr>

 * </table>

 *