DEV: Updated docstrings for nor and sub.
DEV: Updated docstrings for nor and sub.

Changed input params to x1 and x2. Provided complete examples
for both logical_nor and logical_sub. The docstrings and functions
are now written to match those provided in numpy.

tacaswell/scikit-xray,CJ-Wright/scikit-beam,licode/scikit-beam,celiafish/scikit-xray,licode/scikit-xray,licode/scikit-beam,tacaswell/scikit-beam,giltis/scikit-xray,tacaswell/scikit-beam,licode/scikit-xray,ericdill/scikit-xray,CJ-Wright/scikit-beam,licode/scikit-xray,hainm/scikit-xray,tacaswell/scikit-beam,danielballan/scikit-xray,Nikea/scikit-xray,celiafish/scikit-xray,tacaswell/scikit-xray,scikit-xray/scikit-xray,danielballan/scikit-xray,yugangzhang/scikit-beam,CJ-Wright/scikit-beam,celiafish/scikit-xray,ericdill/scikit-xray,scikit-xray/scikit-xray,yugangzhang/scikit-beam,ericdill/scikit-xray,danielballan/scikit-xray,scikit-xray/scikit-xray,hainm/scikit-xray,yugangzhang/scikit-beam,licode/scikit-beam,Nikea/scikit-xray,tacaswell/scikit-xray,Nikea/scikit-xray,giltis/scikit-xray,hainm/scikit-xray,giltis/scikit-xray