Misc documentation errors

[sbromberger]

All in http://graphsjl-docs.readthedocs.org/en/latest/interface.html#basic-interface

I'll try to fix these up later but if someone wants a go at them in the meantime, feel free :)

1. http://graphsjl-docs.readthedocs.org/en/latest/interface.html#edge_index should refer to edges, not vertices.
2. in_edges and out_edges don't return the number of edges, they return iterators of edges.
3. source() and target() are defined twice

[mlubin]

I think the repetition of source() and target() is okay because they're part of two different interfaces.