refine prelu api doc, test=develop

python/paddle/fluid/layers/nn.py

@@ -6900,14 +6900,14 @@ def prelu(x, mode, param_attr=None, name=None):
     """
     Equation:
 
-        y = \max(0, x) + alpha \min(0, x)
+        y = \max(0, x) + alpha * \min(0, x)
 
     Args:
         x (Variable): The input tensor.
 	  param_attr(ParamAttr|None): The parameter attribute for the learnable
                                       weight (alpha).
-        mode (string): The mode for weight sharing
-		       all: all elements share same weight
+        mode (string): The mode for weight sharing. It supports all, channel
+                       and element. all: all elements share same weight
                        channel:elements in a channel share same weight
                        element:each element has a weight
 	name(str|None): A name for this layer(optional). If set None, the layer