[DOC] Clarify that global pooling is going to reset padding

This behaviour changed from older MXNet versions in which global pooling
would consider padding. This clarifies the user documentation.

See also apache#14421

src/operator/nn/pooling.cc

@@ -389,8 +389,8 @@ The definition of *f* depends on ``pooling_convention``, which has two options:
 
     f(x, k, p, s) = ceil((x+2*p-k)/s)+1
 
-But ``global_pool`` is set to be true, then do a global pooling, namely reset
-``kernel=(height, width)``.
+When ``global_pool`` is set to be true, then global pooling is performed. It will reset
+``kernel=(height, width)`` and set the appropiate padding to 0.
 
 Three pooling options are supported by ``pool_type``: