fix lstm bias_attr doc

test=release/1.0.0

Author: chengduozh

python/paddle/fluid/layers/nn.py

@@ -351,7 +351,6 @@ def dynamic_lstm(input,
         c_0(Variable): The initial cell state is an optional input, default is zero.
                        This is a tensor with shape (N x D), where N is the
                        batch size. `h_0` and `c_0` can be NULL but only at the same time.
-
         param_attr(ParamAttr|None): The parameter attribute for the learnable
                                hidden-hidden weights.
 
@@ -3131,10 +3130,18 @@ def lstm_unit(x_t,
         cell_t_prev (Variable): The cell value of lstm unit, a 2-D tensor with
             shape M x S, M for batch size and S for size of lstm unit.
         forget_bias (float): The forget bias of lstm unit.
-        param_attr (ParamAttr): The attributes of parameter weights, used to set
-            initializer, name etc.
-        bias_attr (ParamAttr): The attributes of bias weights, if not False,
-            bias weights will be created and be set to default value.
+        param_attr(ParamAttr|None): The parameter attribute for the learnable
+                               hidden-hidden weights.
+                               If it is set to None or one attribute of ParamAttr,
+                               lstm_unit will create ParamAttr as param_attr.
+                               If the Initializer of the param_attr is not set, the
+                               parameter is initialized with Xavier. Default: None.
+        bias_attr (ParamAttr|None): The bias attribute for the learnable bias
+                              weights. If it is set to False, no bias will be added
+                              to the output units. If it is set to None or one attribute of ParamAttr,
+                              lstm_unit will create ParamAttr as bias_attr.
+                              If the Initializer of the bias_attr is not set,
+                              the bias is initialized zero. Default: None.
         name(str|None): A name for this layer(optional). If set None, the layer
                        will be named automatically.