Update docs

Summary: Improve documentation

Reviewed By: ppwwyyxx

Differential Revision: D19779790

fbshipit-source-id: 83532d40ea14d23700328f5cea956fff479356a0

Author: Alexander Kirillov

projects/PointRend/configs/InstanceSegmentation/Base-PointRend-RCNN-FPN.yaml

@@ -17,4 +17,5 @@ MODEL:
     NUM_FC: 3
     IN_FEATURES: ["p2"]
 INPUT:
+  # PointRend for instance segmenation does not work with "polygon" mask_format.
   MASK_FORMAT: "bitmask"