Use meaningful names in txk-sel rd control

Change-Id: I83ca47c1469d8e383a815058c02c4826c6282873
diff --git a/av1/encoder/block.h b/av1/encoder/block.h
index 501b21b..0b334cf 100644
--- a/av1/encoder/block.h
+++ b/av1/encoder/block.h
@@ -164,6 +164,9 @@
   // Save the transform RD search info.
   TX_RD_RECORD tx_rd_record;
 
+  // Determine if one would go with reduced complexity transform block
+  // search model to select prediction modes, or full complexity model
+  // to select transform kernel.
   int rd_model;
 
   // Also save RD info on the TX size search level for square TX sizes.