Add high level documentation to handle_inter_mode

Gives a high level description of what the function and main loop
do. More low level documentation will be added throughout the function
in a followup.

BUG=aomedia:2615

Change-Id: Ieed8181488cf218dbd558e1e79d29ca74c523b84

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index ab0c54b..655d0de 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -2358,6 +2358,12 @@
   return 0;
 }
 
+/*
+   Do the RD search for a given inter mode and compute all information relevant
+   to the input mode. It will compute the best MV,
+   compound parameters (if the mode is a compound mode) and interpolation filter
+   parameters.
+*/
 static int64_t handle_inter_mode(
     AV1_COMP *const cpi, TileDataEnc *tile_data, MACROBLOCK *x,
     BLOCK_SIZE bsize, RD_STATS *rd_stats, RD_STATS *rd_stats_y,
@@ -2436,6 +2442,15 @@
   const int ref_mv_cost = cost_mv_ref(x, this_mode, mode_ctx);
   const int base_rate =
       args->ref_frame_cost + args->single_comp_cost + ref_mv_cost;
+
+  // Main loop of this function. This will  iterate over all of the ref mvs
+  // in the dynamic reference list and do the following:
+  //    1.) Get the current MV. Create newmv MV if necessary
+  //    2.) Search compound type and parameters if applicable
+  //    3.) Do interpolation filter search
+  //    4.) Build the inter predictor
+  //    5.) Pick the motion mode (SIMPLE_TRANSLATION, OBMC_CAUSAL,
+  //    WARPED_CAUSAL) 6.) Update stats if best so far
   for (int ref_mv_idx = 0; ref_mv_idx < ref_set; ++ref_mv_idx) {
     mode_info[ref_mv_idx].full_search_mv.as_int = INVALID_MV;
     mode_info[ref_mv_idx].mv.as_int = INVALID_MV;