add comments, test=develop

paddle/fluid/operators/detection/box_coder_op.cc

@@ -167,6 +167,10 @@ and height respectively. Similarly, `px`, `py`, `pw`, `ph` denote the
 priorbox's (anchor) center coordinates, width and height. `pxv`, `pyv`, `pwv`,
 `phv` denote the variance of the priorbox and `ox`, `oy`, `ow`, `oh` denote the
 encoded/decoded coordinates, width and height. 
+
+During Box Decoding, two modes for broadcast are supported. Say target box has 
+shape [N, M, 4], and the shape of prior box can be [N, 4] or [M, 4]. Then prior
+box will broadcast to target box along the assigned axis. 
 )DOC");
   }
 };