Update documentation for gradient of `clip`

theano/tensor/basic.py

@@ -2990,11 +2990,16 @@ def pow(a, b):
     # see decorator for function body
 
 
-# The numpy.clip don't work correctly when
-# the min is bigger then the max
-@_scal_elemwise  # _with_nfunc('clip', 3, 1)
+# The numpy.clip don't work correctly when the min is bigger then the max,
+# So we do not use @scal_elemwise_with_nfunc('clip', 3, 1)
+@_scal_elemwise
 def clip(x, min, max):
-    """clip x to be between min and max"""
+    """clip x to be between min and max.
+
+    :note: When `x` is equal to the boundaries, the output is considered
+        to be `x`, so at these points, the gradient will flow through `x`,
+        not through `min` nor `max`.
+    """
     # see decorator for function body
     # for grep: clamp, bound