fix doxygen problems

Change-Id: If663aafa028e327a7b87004ee58f4f106341b8af

vpx/internal/vpx_codec_internal.h

@@ -240,7 +240,7 @@ typedef vpx_image_t *(*vpx_codec_get_frame_fn_t)(vpx_codec_alg_priv_t *ctx,
  *
  * \note
  * When decoding VP9, the application may be required to pass in at least
- * #VP9_MAXIMUM_REF_BUFFERS + #VPX_MAXIMUM_WORK_BUFFERS external frame
+ * #VPX_MAXIMUM_WORK_BUFFERS external frame
  * buffers.
  */
 typedef vpx_codec_err_t (*vpx_codec_set_fb_fn_t)(

vpx/vpx_decoder.h

@@ -360,7 +360,7 @@ extern "C" {
    *
    * \note
    * When decoding VP9, the application may be required to pass in at least
-   * #VP9_MAXIMUM_REF_BUFFERS + #VPX_MAXIMUM_WORK_BUFFERS external frame
+   * #VPX_MAXIMUM_WORK_BUFFERS external frame
    * buffers.
    */
   vpx_codec_err_t vpx_codec_set_frame_buffer_functions(