refine docs of elementwise_op etc.

Author: luotao1

paddle/fluid/operators/elementwise_op.h

@@ -72,23 +72,24 @@ The equation is:
 
 $$%s$$
 
-$X$ is a tensor of any dimension and the dimensions of tensor $Y$ must be
-smaller than or equal to the dimensions of $X$.
+$X$ is a tensor of any dimension. And $Y$ is a tensor whose dimensions must be
+less than or equal to the dimensions of $X$.
 
 There are two cases for this operator:
-1. The shape of $Y$ is same with $X$;
-2. The shape of $Y$ is a congiguous subsequencet of $X$. The trailing dimensions
-   of size 1 for $Y$ will be ignored for the consideration of subsequence.
 
+1. The shape of $Y$ is the same with $X$.
+2. The shape of $Y$ is a continuous subsequence of $X$.
 
 For case 2:
 
-$Y$ will be broadcasted to match the shape of $X$ and axis should be
-set to index of the start dimension to broadcast $Y$ onto $X$.
+1. Broadcast $Y$ to match the shape of $X$, where $axis$ is the start dimension index 
+   for broadcasting $Y$ onto $X$. 
+2. If $axis$ is -1 (default), $axis = rank(X) - rank(Y)$.
+3. The trailing dimensions of size 1 for $Y$ will be ignored for the consideration of 
+   subsequence, such as shape(Y) = (2, 1) => (2).
 
-If axis is -1, it is treated as axis=rank(X)-rank(Y).
+For example:
 
-For example
   .. code-block:: python
 
     shape(X) = (2, 3, 4, 5), shape(Y) = (,)
@@ -98,8 +99,8 @@ For example
     shape(X) = (2, 3, 4, 5), shape(Y) = (2), with axis=0
     shape(X) = (2, 3, 4, 5), shape(Y) = (2, 1), with axis=0
 
-Either of the inputs $X$ and $Y$ or none can carry the LoD (Level of Details)
-information. However, the output only shares the LoD information with input $X$.
+The inputs $X$ and $Y$ can carry the different LoD information. 
+But the output only shares the LoD information with the input $X$.
 
 )DOC",
                                GetName(), GetEquation()));

python/paddle/fluid/layers/nn.py

@@ -1409,7 +1409,7 @@ def sequence_pool(input, pool_type):
 
 def sequence_first_step(input):
     """
-    This funciton get the first step of sequence.
+    This function get the first step of sequence.
 
     .. code-block:: text
 
@@ -1442,7 +1442,7 @@ def sequence_first_step(input):
 
 def sequence_last_step(input):
     """
-    This funciton get the last step of sequence.
+    This function get the last step of sequence.
 
     .. code-block:: text