BUG: initialize op_axes when only itershape is given

In this case, the initialization used the number of dimensions instead
of setting it to NULL for each operand, leading to possible segmentation
faults.
DOC: Remove documentation of non-existing advanced iter NA flags.
Merge pull request #3105 from seberg/backport-nditer-fix-op_axes-initialization

Backport nditer fix op axes initialization