Change of docs for shunts in network data format

[LKuhrmann]

Changed the network data format shunt description from

"gs":<float>,      # Active power withdrawn per voltage p.u.
"bs":<float>,      # Reactive power withdrawn per voltage p.u.

to

"gs":<float>,      # Active power withdrawn at unity voltage (1 p.u.)
"bs":<float>,      # Reactive power injected at unity voltage (1 p.u.)

Reduces the confusion from #978.

Also changed from per voltage p.u. to at unity voltage (1 p.u.) as the current phrasing implies a false linearity with voltage (it is quadratic).