Added the numpy implementation of switch to the docs. (keras-team#11575)

### Summary

I used `np.newaxis` for users who may not be familiar with the `None` indexing.

### Related Issues

### PR Overview

- [ ] This PR requires new unit tests [y/n] (make sure tests are included)
- [ ] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
- [x] This PR is backwards compatible [y/n]
- [ ] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

keras/backend/numpy_backend.py

@@ -277,7 +277,7 @@ def relu(x, alpha=0., max_value=None, threshold=0.):
 def switch(condition, then_expression, else_expression):
     cond_float = condition.astype(floatx())
     while cond_float.ndim < then_expression.ndim:
-        cond_float = cond_float[..., None]
+        cond_float = cond_float[..., np.newaxis]
     return cond_float * then_expression + (1 - cond_float) * else_expression
 
 

keras/backend/tensorflow_backend.py

@@ -3227,6 +3227,8 @@ def switch(condition, then_expression, else_expression):
 
     # Raises
         ValueError: If rank of `condition` is greater than rank of expressions.
+
+    {{np_implementation}}
     """
     if condition.dtype != tf.bool:
         condition = tf.cast(condition, 'bool')