[core API doc fixit] tf.math.add; updated the API doc with more explanations and examples.

PiperOrigin-RevId: 345120053
Change-Id: If3df40d44a1ca78129a56f2c8c5ca33e98a5bd1e

Author: hyeygit

tensorflow/core/api_def/python_api/api_def_Add.pbtxt

@@ -6,4 +6,62 @@ op {
   endpoint {
     name: "add"
   }
+  description: <<END
+Example usages below.
+
+Add a scalar and a list:
+
+>>> x = [1, 2, 3, 4, 5]
+>>> y = 1
+>>> tf.add(x, y)
+<tf.Tensor: shape=(5,), dtype=int32, numpy=array([2, 3, 4, 5, 6], dtype=int32)>
+
+Note that binary `+` operator can be used instead:
+
+>>> x = tf.convert_to_tensor([1, 2, 3, 4, 5])
+>>> y = tf.convert_to_tensor(1)
+>>> x + y
+<tf.Tensor: shape=(5,), dtype=int32, numpy=array([2, 3, 4, 5, 6], dtype=int32)>
+
+Add a tensor and a list of same shape:
+
+>>> x = [1, 2, 3, 4, 5]
+>>> y = tf.constant([1, 2, 3, 4, 5])
+>>> tf.add(x, y)
+<tf.Tensor: shape=(5,), dtype=int32,
+numpy=array([ 2,  4,  6,  8, 10], dtype=int32)>
+
+**Warning**: If one of the inputs (`x` or `y`) is a tensor and the other is a
+non-tensor, the non-tensor input will adopt (or get casted to) the data type of
+the tensor input. This can potentially cause unwanted overflow or underflow
+conversion.
+
+For example,
+
+>>> x = [2**7 + 1, 2**7 + 2]
+>>> y = tf.constant([1, 2], dtype=tf.int8)
+>>> tf.add(x, y)
+<tf.Tensor: shape=(2,), dtype=int8, numpy=array([-126, -124], dtype=int8)>
+
+When adding two input values of different shapes, `Add` follows the [general
+broadcasting rules](https://numpy.org/doc/stable/user/basics.broadcasting.html#general-broadcasting-rules)
+. The two input array shapes are compared element-wise. Starting with the
+trailing dimensions, the two dimensions either have to be equal or one of them
+needs to be `1`.
+
+For example,
+
+>>> x = np.ones(6).reshape(1, 2, 1, 3)
+>>> y = np.ones(6).reshape(2, 1, 3, 1)
+>>> tf.add(x, y).shape.as_list()
+[2, 2, 3, 3]
+
+Another example with two arrays of different dimension.
+
+>>> x = np.ones([1, 2, 1, 4])
+>>> y = np.ones([3, 4])
+>>> tf.add(x, y).shape.as_list()
+[1, 2, 3, 4]
+
+END
 }