Clarifications for multi-output networks

[jonathanstrong]

A few additions to clarify how multi-output networks should be handled, per the discussion here: https://groups.google.com/forum/#!topic/lasagne-users/61GbgRqKjAo

[f0k]

Thanks! Some of the lines are too long, though, we have a hard limit of 79 characters to comply with PEP8. (Not your fault, you couldn't tell when using the web interface.)

And I'm not sure where to put the clarification, actually. When re-reading the existing docstrings, they already say thing like "Its main use is to collect all layers of a network just given the output layer(s)." -- so they advertise supplying all output layers. The first sentence of get_all_params should probably say network instead of layer, though. What do you think? Is there a less verbose way of clarifying things such that you'd have known what to do?