Fix the doc of PyArray_ArgMax and PyArray_ArgMin
Avoid overallocating memory in arraypad with user supplied function

Previously, arraypad used zeros(shape).astype(dtype) instead of
zeros(shape, dtype), which could allocate up to eight times more memory
than necessary.
Merge pull request #4128 from longjon/master

Avoid overallocating memory in arraypad with user supplied function
Merge pull request #4127 from nouiz/argmax

DOC: Fix doc and error handling in Argmax