Polish API doc

test=develop

python/paddle/fluid/layers/nn.py

@@ -5830,11 +5830,12 @@ 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 RankNet**
+    **Margin Rank loss layer for rank problem**
      Args:
-        label (Variable): Indicats whether A ranked higher than B or not.
+        label (Variable): Indicats whether left higher than (right + margin) or not.
-        left (Variable): RankNet's output score for doc A.
+        left (Variable): rank score for left.
-        right (Variable): RankNet's output score for doc B.
+        right (Variable): rank score for right.
+        margin (float): Indicates the margin to be added to right
         name (str|None): A name for this layer (optional). If set None, the layer
                         will be named automatically.
      Returns: