Add docs about threading to SurfaceTexture.

/**

 * Captures frames from an image stream as an OpenGL ES texture.

 *

 * <p>The image stream may come from either video playback or camera preview.  A SurfaceTexture may

 * be used in place of a SurfaceHolder when specifying the output destination of a MediaPlayer or

 * Camera object.  This will cause all the frames from that image stream to be sent to the

 * <p>The image stream may come from either camera preview.  A SurfaceTexture may be used in place

 * of a SurfaceHolder when specifying the output destination of a {@link android.hardware.Camera}

 * object.  Doing so will cause all the frames from the image stream to be sent to the

 * SurfaceTexture object rather than to the device's display.  When {@link #updateTexImage} is

 * called, the contents of the texture object specified when the SurfaceTexture was created is

 * updated to contain the most recent image from the image stream.  This may cause some frames of

 *

 * <p>The texture object uses the GL_TEXTURE_EXTERNAL_OES texture target, which is defined by the

 * OES_EGL_image_external OpenGL ES extension.  This limits how the texture may be used.

 *

 * <p>SurfaceTexture objects may be created on any thread.  {@link #updateTexImage} may only be

 * called on the thread with the OpenGL ES context that contains the texture object.  The

 * frame-available callback is called on an arbitrary thread, so unless special care is taken {@link

 * #updateTexImage} should not be called directly from the callback.

 */

public class SurfaceTexture {