Polish doc

test=develop

python/paddle/fluid/layers/nn.py

@@ -5830,7 +5830,13 @@ def rank_loss(label, left, right, name=None):
 
 def margin_rank_loss(label, left, right, margin=0.1, name=None):
     """
-    **Margin Rank loss layer for rank problem**
+    Margin Rank loss layer for rank problem, which comparing left value and right value be passed in.
+    The rank loss can be defined as below equation:
+
+    .. math::
+
+        rank\_loss &= max(0, -label * (left - right) + margin)
+
     Args:
        label (Variable): Indicats whether left higher than (right + margin) or not.
        left (Variable): rank score for left.