Updates set_{node,edge}_attributes and docs.
Updates set_{node,edge}_attributes and docs.

This commit simplifies `set_node_attributes` and `set_edge_attributes`
by using `zip_longest` in order to avoid creating a list when not
necessary. It also clarifies the documentation.

jfinkels/networkx,JamesClough/networkx,goulu/networkx,cmtm/networkx