Add proper Oxygen comments for MV trajectory tracking related fields

Fixes #474
diff --git a/av1/common/av1_common_int.h b/av1/common/av1_common_int.h
index b84903b..6c2cb97 100644
--- a/av1/common/av1_common_int.h
+++ b/av1/common/av1_common_int.h
@@ -1685,11 +1685,20 @@
   TPL_MV_REF *tpl_mvs;
 
 #if CONFIG_TMVP_MEM_OPT
+  /*!
+   * Step size for tmvp sampling. Should be 1 (no sampling) or 2.
+   */
   int tmvp_sample_step;
 #endif  // CONFIG_TMVP_MEM_OPT
 
 #if CONFIG_MV_TRAJECTORY
+  /*!
+   * Mapping table from trajectory id to the offset to the current block.
+   */
   int_mv *id_offset_map[INTER_REFS_PER_FRAME];
+  /*!
+   * Mapping table from block location to trajectory id.
+   */
   int *blk_id_map[INTER_REFS_PER_FRAME];
 #endif  // CONFIG_MV_TRAJECTORY